The Romanian, a two-time runner-up at Roland Garros, found herself 5-0 down to American Alison Riske in Wednesday’s opening match on Court Philippe Chatrier.
Riske went on to take the first set but Halep finally woke up and dropped just two more games in registering a 2-6, 6-1, 6-1 victory.
Halep, still hunting a maiden grand slam title, will face another American, Taylor Townsend, in round two.
Halep refused to blame the rain delay, which prevented her from playing Tuesday evening, for her sluggish beginning.
“I had a slow start because it’s always tough to start this tournament,” she said.
“It’s a pleasure to come here and to play, so always I feel nervous at the beginning.
“But it was good that I came back so strong. I felt really well in the end of the match, and that’s the most important for now.”
In the second round, fourth seed Elina Svitolina, of Ukraine, breezed past Slovakian Viktoria Kuzmova 6-3, 6-4.
Petra Kvitova, the Czech eighth seed, was untroubled by Lara Arruaberra of Spain in a 6-0, 6-4 victory to take her into the third round, where she will face Estonia’s Anett Kontaveit.
The Serbian, seeded only 20 due to his recent injury absence, will now face Spanish 13th seed Roberto Bautista Agut after a straight-sets win over Jaume Munar.
Munar may be ranked a lowly 155 in the world but he made Djokovic work for every point in some lengthy and at times spectacular rallies.
The Spaniard, broken early on, broke back in the opening set only for Djokovic to race through the tie-break 7-1.
Munar occasionally had Djokovic scrambling throughout the next two sets with his fearsome shot-making, but the 2016 champion was able to post a 7-6 (7/1) 6-4 6-4 victory.
In a match held over from Tuesday Tomas Berdych, the 17th seed, was a first-round casualty after he was beaten by France’s Jeremy Chardy in five sets.
Czech Berdych, a former Wimbledon finalist, came from two sets down to haul himself level only to bow out 7-6 (7/5) 7-6 (10/8) 1-6 5-7 6-2.
Meanwhile, Marco Trungelliti’s epic journey came to an end in the second round.
The Argentinian became the unlikely darling of Roland Garros after driving for 10 hours from Barcelona to Paris, with his brother, mother and gran, on Sunday in order to register in time as a lucky loser.
Trungelliti, the world number 190, then stunned the tennis world by beating former top-20 player Bernard Tomic to net himself £69,000 in prize money.
However, the 28-year-old hit the skids in round two as he was beaten 6-1 7-6 (7/1) 6-1 by Italian Marco Cecchinato.
The former world number one had not played a major tournament since claiming a 23rd title at the 2017 Australian Open, having given birth to her daughter, Olympia, in September.
Her first grand slam action in over 15 months had the Paris fashionistas in a whirl as she walked on court in a black bodysuit with a bright pink belt.
Afterwards Williams revealed: “I feel like a warrior in it, like a warrior princess, kind of.
“I’m always living in a fantasy world. I always wanted to be a super hero, and it’s kind of my way of being a super hero. I feel like a super-hero when I wear it.”
This was not quite a super hero performance, to be expected with Williams short of match sharpness and her footwork not quite there yet.
But the 36-year-old’s serve remains a formidable weapon, helping her take the first set after a tie-break.
The pair exchanged breaks in the second set, with Williams grabbing the decisive one in the seventh game before sealing a 7-6, (7/4) 6-4 victory – not bad going for her first match on clay in two years.
Maria Sharapova, meanwhile, lost six games in a row and then won the next six as she got her return to Roland Garros off to a successful start.
The Russian appeared to be cruising at 6-1, 3-1 ahead but Holland’s Richel Hogenkamp suddenly had her on the ropes, taking the second set and racing 3-0 ahead in the decider.
But Sharapova rediscovered her game in the nick of time to reel off half a dozen games for a 6-1, 4-6, 6-3 victory.
It was a sweet moment for the two-time Paris champion.
This time last year Sharapova had recently returned from a 15-month doping ban, but had not built her ranking high enough to qualify and was not offered a wild card.
But 12 months later Sharapova is back in the world’s top 30, seeded 28 at Roland Garros and a potential challenger for a third title.
“I think if there is any way to turn that match around, it’s that way,” she said.
“It’s great to be in this draw and great to be back on a court that I have had great success at.”
Wimbledon champion Garbine Muguruza got the better of Svetlana Kuznetsova in a first-round meeting of two former winners.
Caroline Garcia of France, the seventh seed, took just 58 minutes to dispatch China’s Yingying Duan 6-1, 6-0.
In the men’s section, Rafael Nadal began his campaign for an 11th French Open title with a rain-delayed 6-4, 6-3, 7-6 (11-9) win over Italy’s Simone Bolelli – his 80th victory at Roland Garros.
Resuming 3-0 down in the third set, Nadal swiftly recovered the break but was pushed hard by the world number 129.
The Spaniard saved three set points from 6-3 down in the tie-break before prevailing in damp conditions.
Nadal will play Argentina’s Guido Pella in the second round.