After Saints missed a couple of chances, the Reds went in front on the stroke of half-time when Mane curled a superb effort into the top corner against his former club.
Firmino added a second 19 minutes from time before ex-Liverpool striker Danny Ings profited from a mistake by Adrian, who was fit enough to start despite an ankle issue, to set up a nervy finish.
Here are three talking points from St Mary’s.
INDIVIDUAL QUALITY MATTERS
If there is one trait Liverpool have developed over the last 12 months, it’s that they are merciless.
Take the game as a whole, and Jurgen Klopp’s men did not merit three points at St Mary’s.
The difference was in the details. Southampton in the first half were far better as Liverpool’s exhausting exploits on Wednesday in the Super Cup extended to Saturday.
A lethargy infected the Reds and Southampton needed no invitation to feed off their inertia, charging high up the pitch and forcing the visitor’s into imprecision on the ball.
For large parts of the opening half, Liverpool struggled to get out of their own half as the front three was left completely isolated.
But during this period of dominance, Southampton did not have any tangible success. Maya Yoshida was denied by Adrian’s superb close-range stop after heading James Ward-Prowse’s delicious corner.
There was panic in the box when Ryan Bertrand’s dangerous low cross pinged across the area as Adrian feathered the ball into a swarm of Southampton attackers, while striker Che Adams also had a sight of goal only to head over.
All that was missing was a touch of quality and Liverpool have exactly that. It takes only a moment for it to be effective.
In tight space on the left, a series of one-touch passes between Andy Robertson, James Milner and Mane opened Southampton up and the Senegalese shifted inside to unleash a gorgeous strike into the far corner.
It was a ruthless passage of play right before half-time. After the break, Southampton’s press lost steam and as their energy levels dropped, the opportunity to prosper narrowed.
Liverpool went on the hunt and Firmino’s quality also told, his drilled effort killing the game off. Or so they thought…
ADRIAN’S ROCKY MOMENT
Jurgen Klopp was hailing back-up goalkeeper Adrian’s “Rocky moment” after his Super Cup heroics.
On Saturday, there were plenty more rocky moments, only this time, the Spaniard’s new colleagues were left screaming in pure anger.
Naturally, the focus of their vex will zero in on the howler which let Southampton back into the game. It’s a scenario which has played out hundreds of times with Alisson, the defender in possession finding no route forward and turning to start again from the keeper. Virgil van Dijk calmly played it back to Adrian from the left, the goalkeeper was unrushed and effortlessly passed it into his own net off Ings.
It wasn’t the first sedate moment either with Adrian very fortunate in the first-half when Ward-Prowse mirrored Ings only for the ball to go harmlessly out for a goalkick.
His distribution was generally quite poor, a passing accuracy of 75.9 percent more than five percent down on Alisson’s average in 2018/19.
However, the Brazilian too had his own faux pas early on last season against Leicester and perhaps Adrian should be afforded the same grace he was, especially considering the mistake didn’t effect the points haul.
OX IS ACTUALLY LIKE A NEW SIGNING
Liverpool fans spent the summer in a meltdown and it had nothing to do with the temperature outside.
Indeed, the club’s inactivity in the transfer window left many anxious going into the new season and that wasn’t aided by Klopp labelling Alex Oxalde-Chamberlain and Adam Lallana as “like new signings”.
Although supporters’ ire can be forgiven in regards to the latter, in Oxlade-Chamberlain’s case it’s a legitimate assessment.
The 26-year-old was excellent on Saturday. Emboldened in the middle three after an unsuccessful outing in the front trio on Wednesday, the England international gave Liverpool a drive through the middle which has been missed.
In the first minute, he bounded through the centre with bowling-ball-like qualities, carrying possession to the edge of Southampton’s box and slipping in Trent Alexander-Arnold on the right to create an opening.
There was a superb piece of combination play with Salah which ended with a sumptuous first-time cross from the right byline.
He created chances and stopped them too, covering for Alexander-Arnold after the right-back was caught out high up the opposite end with Oxlade-Chamberlain walling in Nathan Redmond to shut down an avenue to goal.
The former Arsenal man was explosive in pockets and tidy with his passing as well, a total contrast to the rest of Liverpool’s midfield.
Liverpool manager Jurgen Klopp has criticised the behaviour of a pitch invader after an accidental collision with Adrian left the goalkeeper with an ankle injury and a doubt for the trip to Southampton.
Adrian marked his first start for the Reds with the decisive save from Chelsea’s Tammy Abraham in the Super Cup final penalty shootout on Wednesday, but as he celebrated with his team-mates, a supporter slid into the Spaniard.
The 32-year-old summer signing is deputising for Alisson Becker, sidelined for a number of weeks because of a calf complaint, meaning Klopp may be forced to deploy 35-year-old third-choice Andy Lonergan on Saturday.
When asked whether the former West Ham shot-stopper’s shootout heroics in Istanbul would boost his confidence, Klopp responded: “It would help him a lot if it would be 100 per cent clear he can play tomorrow.
“A supporter jumped over something, was chased by some security guys, slipped and kicked his ankle. It’s crazy.
“There’s no doubt about how much we love our fans but if they could all stop doing that. We played against City, somebody was running on the pitch? And Norwich there was someone as well.
“The girl in the Champions League final made money from it, I don’t know what you can do against it.
“I like the fact that we don’t have the fences in the stadiums but that means there is a lot of responsibility for the supporters as well.
“If the one person cannot hesitate or stop himself then the people around should do it. It’s not funny. It’s crazy, you see it and you think ‘wow, how can something like this happen?’”
Videos on social media showed a fan slipping into a group of huddled-together Liverpool players after being grabbed by a steward. UEFA confirmed to the PA news agency it was in contact with Liverpool over the matter, but had no further comments to make at this stage.
Elaborating on Adrian’s ankle, Klopp added: “Yesterday it was swollen, today I spoke to him and he said ‘it’s much better than yesterday’ but we have to see.
“He will not go for a scan. It went down already. You can imagine you have that and you fly four hours back and it doesn’t help swelling, so he couldn’t have played directly after we landed in Liverpool.
“But I saw him now, it’s OK and it looks better. But a decision will be made tomorrow, no chance to make it earlier.”
Alisson’s injury prompted Liverpool to sign Lonergan on a short-term contract, the former Preston, Leeds and Bolton goalkeeper having trained with the Merseysiders over the summer.
Asked whether Lonergan would be ready to step in, Klopp responded: “Yeah. (Caoimhin) Kelleher is now in training as well, so we have them. If we have to make a decision, we have to make a decision.”
Klopp, who will definitely be without midfielder Naby Keita because of a muscle strain, refused to be drawn on his side’s demanding schedule, with their visit to the south coast coming three days after the Super Cup final in Turkey.
He added: “My plan when I heard about the fixtures was to win on Wednesday, to win on Saturday and to speak about it after that because if you speak about it earlier, it sounds you’re looking for excuses. I don’t, we don’t, it’s like it is.
“Was it right that we played on Friday and Chelsea on Sunday? No. The other things we cannot change.
“We cannot change these things, we have no power, absolutely nil power. We only have to accept it like it is and that’s what we do.”
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Istanbul is already a special place for the Reds following their stunning Champions League triumph against AC Milan in 2005, and once again the Turkish city played host to a continental success for the Anfield giants.
Olivier Giroud put Chelsea ahead in the first half, only for Sadio Mane to level as the match went to extra-time. Mane scored again before Adrian brought Tammy Abraham down in the box, allowing Jorginho to make it 2-2.
Just as against Milan 14 years ago, it took penalties to split the sides as Adrian wrote his name into folklore at a club he only joined at the start of last week.
“What a story – Adrian. Like Rocky,” Klopp told BT Sport, referencing the film released in 1977 when the underdog fighter cries out to his girlfriend Adrian after going 15 rounds with heavyweight champion Apollo Creed.
The former West Ham goalkeeper was handed his first Liverpool start in the Super Cup due to Alisson Becker’s calf injury and the Spaniard denied Abraham to secure a 5-4 spot-kick success.
ANOTHER PENALTY SHOOTOUT WIN IN ISTANBUL!!— Liverpool FC (@LFC) August 14, 2019
SUPER CUP CHAMPIONS!!! pic.twitter.com/sZaxIfK9hY
“I don’t know where he was two weeks ago when we were playing Manchester City,” Klopp said at the post-match press conference, referring to the Reds’ Community Shield shootout loss.
“When I spoke to him the first time, it was clear he would take time to get fit. But he has to be fit now and he is.
“He had an incredible game and both goalkeepers made sensational saves, but with Adrian it’s a bit more surprising because it was without pretty much any preparation.
“I don’t know when he played his last game for West Ham, being ready like he was incredible.
“He has already shown me he is a proper personality in the dressing room as well – he was maybe even louder than I was at half-time. He was really on his toes, and that helps us.
“Making the save from the penalty is the icing on the cake. I saw he even had time to grab his towel before celebrating, so he’s obviously quick in mind. He has helped us a lot and can be really proud what he did tonight.”
The win was not without controversy as pictures showed the goalkeeper off his line before saving Abraham’s penalty.
That subject did not come up in the post-match press conference, with Klopp hugely impressed by the performance of referee Stephanie Frappart, who became the first female official to take charge of a major UEFA men’s competition event.
“I spoke to the ref team after the game that if we had played like they whistled, then we would have won 6-0,” the Reds boss said.
“They played a brilliant game. I also said I wasn’t happy with the penalty decision as I’m still not sure but it’s not important any more.
“They were good, they were really good. There was pressure on them like hell. It was an historic moment and they stayed themselves, stayed calm and did what they had to do in a very difficult and intense game.
“I couldn’t have more respect to be honest. It was a really brilliant performance.”
Meanwhile, Chelsea counterpart Frank Lampard told Abraham “not to worry” after his missed spot-kick proved the difference.
“I have been there, missed penalties. Anyone can,” Lampard said.
“I want the confidence of a young player to stand up and want to do that. I said not to worry. He will come back stronger.
“It is part and parcel of being a player in the top level, which Tammy now is. These moments come.
“We have to be very supportive of him. His team-mates were, I certainly am. That is football.”
It was a galling loss fresh on the back of Sunday’s 4-0 humbling at Manchester United, but recently-appointed manager Lampard remained upbeat.
“My overall feeling from the game is one of pride in the team’s performance and happiness,” he said.
“I don’t like losing, I’m a terrible loser as you should be at Chelsea. We wanted to win, but we were very unfortunate today.
“I am proud of the players and if that is a sign of what we can do today then this season we can get better and it’s a really good sign.”