The Danish world number two won 6-3, 7-6 (7/2) in 1hr 37min after almost allowing the world number 37 a way back when serving for the match at 5-4, and seemingly in complete control.
Two double faults enabled Mertens, in her first semi-final at this level on her Australian Open debut, to level at 5-5.
Serving to take it to a tiebreak at 5-6 Wozniacki then needed to save three sets points before sealing the match in the tiebreak.
“It means so much to me.” she said after reaching her maiden Australian Open final and her first Grand Slam decider since 2014, where she will play either top seed Simona Halep or the 2016 Melbourne Park champion Angelique Kerber on Saturday.
“I got really tight at 5-4. I thought ‘calm down it’s all good’. It wasn’t good anymore. Served a couple of double faults.
— #AusOpen (@AustralianOpen) January 25, 2018
“Normally I am really calm so once I started feeling really nervous, it felt like my legs were shaking a bit.
“I just took a few deep breaths and once she had set point, I said: ‘Well, I guess it is a third set. Just need to go forward’.”
Wozniacki has never quite lived up to the hype in the majors – this will be just her third Grand Slam final appearance, nine years after her first at the US Open in 2009.
The 27-year-old rose to the top of the world rankings in 2010 but has only made the title match at a Grand Slam once since then, also at Flushing Meadows in 2014.
Mertens, in only her fifth Grand Slam appearance and Australian Open debut, signalled her intent to attack from the start, standing inside the baseline to receive Wozniacki’s second serve.
It was a high-risk strategy and with Wozniacki repelling all the Belgian’s aggressive overtures, the errors began to flow at regular intervals from Mertens’ racket.
Serving at 1-3, 15-40, a netted forehand, her fifth unforced error on that wing, gave the Dane, back in a semi-final at Melbourne for the first time since 2011, the first break.
Mertens, seeking to become the first Belgian since Kim Clijsters here in 2011 to reach a Slam final, kept up the attack and fashioned a break point in the next game but Wozniacki stood up to the challenge and held for 4-1.
Serving at 2-5, Mertens saved a set point when Wozniacki went long with a forehand. The Dane was unperturbed by the minor setback and held to love to seal a comfortable first set 6-3 after 38 minutes.
There was little between the pair in terms of winners in the first stanza, Wozniacki edging the count 13 to 12. But the unforced error count was telling – the Dane committing just six while Mertens threw away 14 points.
The second set went with serve until the Dane’s late wobble, before she finally made it through.
— #AusOpen (@AustralianOpen) January 25, 2018
Provided by AFP Sport
It had been almost 20 years since multiple unseeded players had last reached the semi-finals at the Australian Open. A decade since just one unseeded player featured in the last-four in Melbourne.
This fortnight, world No. 49 Kyle Edmund, and No. 58 Chung Hyeon have made a statement for the younger generation, storming into the semi-finals at Melbourne Park. Unseeded and aged 23 and 21 respectively, Edmund and Chung have impressed with their unlikely runs, bringing with them a breath of fresh air to an ageing tour.
“I think it’s a good thing,” said the 36-year-old Roger Federer on the arrival of the young guns to the big stage. “They got to make a move. I find it disappointing when their breakthroughs come at 27, because then we know them for seven years, let’s say. I like it when we don’t know the guys.
— Reem Abulleil (@ReemAbulleil) January 24, 2018
“I hardly know Chung. I’ve hardly spoken to him. I had one Nike appearance once with Edmund over in London. That’s about it. Maybe otherwise I’ve shaken his hand twice and spoken a few words to him. In a way I like it, because it’s really something totally new to me and to some extent for you guys, too,” he told reporters in Melbourne on Wednesday after defeating Tomas Berdych in straight sets to reach a record-extending 14th Australian Open semi-final.
Federer reminds everyone though that his generation is still far from done.
“It’s not going to happen all the time. We like our rivalries that do exist on the tour. New names are good, from time to time, of course for the tour,” added the Swiss.
They definitely are.
HERO OF THE DAY
Chung’s personality and character continue to shine through Down Under. His straight-sets dismissal of Tennys Sandgren on Wednesday on the heels of his huge upset over six-time champion Novak Djokovic was celebrated for more reasons than one. This was my favourite line from Chung after his quarter-final triumph.
“I think I’m not tired because I win. When I win the match against top player, never tired. Just happy, no? Mentally happy, nothing tired,” said the Next Gen ATP Finals champion.
Spoken like a true star in the making.
STATS OF THE DAY
1 – Simona Halep is the first Romanian woman to reach the semi-finals of the Australian Open.
5 – wins and five losses for Halep in Grand Slam quarter-finals throughout her career.
6 – wins and two losses for Kerber in career Grand Slam quarter-finals.
9 – matches in a row Berdych has now lost to Federer.
10 – years since an unseeded man has made the Australian Open semi-finals (Jo-Wilfried Tsonga in 2008) prior to this fortnight.
14 – Australian Open semi-finals reached by Federer, extending his Open Era record. Stefan Edberg is behind him in second place with eight.
14 – consecutive matches Kerber has now won in 2018, against zero losses.
19 – years since multiple unseeded men have reached the semi-finals of the Australian Open (Lapentti, Haas, Enqvist in 1999) prior to this fortnight. Chung and Edmund have changed that now.
43 – Grand Slam semi-finals Federer has now reached. Another Open Era record.
61 – winners from Federer in his three-set win over Berdych.
QUOTES OF THE DAY
“I don’t know. Maybe he has couple more gray hair.”
— Tomas Berdych when asked what about the difference between the Federer he faced on Wednesday, and the one he faced in Melbourne a year ago.
“I’m evolving, guys (smiling). I’m growing. I think I’m handling myself pretty well right now. We’ll just go with that for today.”
— Madison Keys when told she usually reacts to tough losses in a much more emotional way.
“Seeing what has happened to so many other top seeds here in the draw, I was a bit wary going into tonight.”
— Roger Federer was actually nervous ahead of his match against Tomas Berdych. He’s now beaten him nine times in a row.
“All Asian players looking for Kei and we trying to follow him. He’s the pride of Asian player.”
— Chung pays tribute to Japan’s Kei Nishikori.
The second seed cruised to a 7-6 (1), 6-3, 6-4 win in 2hr 14min on Rod Laver Arena and will face unseeded South Korean Chung Hyeon on Friday for a place in the final.
The 19-time Grand Slam champion, who hasn’t lost in five matches against Berdych in Melbourne, will play in his 43rd Grand Slam semi-final, a record for the post-1968 Open Era.
The Swiss legend has yet to face Chung, who has got past six-time champion Novak Djokovic and world No. 4 Alexander Zverev to get to the last four.
Federer’s latest victory was his 92nd in 105 matches in Melbourne to make the Australian Open his most successful Grand Slam in terms of matches won.
“I’m happy I got out of the first set. It ended up being the key of the match,” he said.
“I just tried to hang around, tried to play a bit more aggressive, get some rhythm going, because Tomas found that early.
“That’s why I was under pressure. It was definitely very close, the turning point, and it ended up being for the entire match.
“I played a great breaker. Got off to a good start there. But coming back from 5-2 in the first set, it was clearly big tonight.”
“It’s great to see new names on the scene… He’s incredibly impressive in his movement. Reminds me a lot of Novak (Djokovic).”
— #AusOpen (@AustralianOpen) January 24, 2018
Federer has now won 14 out of 14 quarter-finals in Melbourne and has yet to drop a set in this year’s tournament.
Looking ahead to his semi-final opponent Chung, Federer said: “He’s incredibly impressive in his movement, he reminds me a lot of Novak the way he is able to slide forehand and backhand and use the hard-court as a clay-court.
“He gets balls back and stays aggressive in defence, so I’m really excited to be playing him, he’s got nothing to lose, I will tell myself the same and we’ll see what happens.”
Federer was broken in his opening service game and fell behind 2-5 before he broke back with a backhand in the ninth game to send the first set into a tiebreaker.
He put the foot down and careered to five set points before taking the tiebreaker with an audacious drop shot.
He continued to put the squeeze on the Czech and broke him in the eighth game and served out for a two sets to love lead after 90 minutes.
Both players exchanged breaks early in the third set before Federer reeled off a signature backhand winner to break again in the fifth game.
He consummately served out for the match for his 20th win in 26 meetings with Berdych.
“I had a good chances, a couple of set points,” Berdych said. “Then he just got more confident after he saved the first set. Then it was very difficult with him again.”
Federer is coming off an extraordinary 2017, when he won a fifth Australian Open title and a record eighth at Wimbledon, after returning from an injury lay-off.
The 36-year-old is bidding to win his 20th Grand Slam title and is the oldest man to reach the semi-finals in Australia since Ken Rosewall (42 years) in 1977.
It was in Melbourne a year ago where he lit the fuse on his late-life tennis renaissance, beating Berdych, Kei Nishikori, Mischa Zverev and Stan Wawrinka before downing great rival Rafael Nadal in a five-set final classic.