Former French Open champion Stan Wawrinka was dumped out in the first round by Guillermo Garcia-Lopez on Monday, a defeat which will send his world ranking plummeting, while late evening rain halted Rafael Nadal's first match in his pursuit of an 11th Roland Garros title.
When play was called off just before 2000 (1800GMT), world number one Nadal was 6-4, 6-3, 0-3 ahead of Italian lucky loser Simone Bolelli.
The downpours also prevented two-time champion Maria Sharapova from starting her opener against Richel Hogenkamp of the Netherlands.
But there were no such problems for 2016 champion Novak Djokovic or Australian Open winner Caroline Wozniacki who both cruised through to the second round before the rains came.
Wawrinka, who lost to Nadal in last year's final and won the 2015 title, was beaten in a five-set thriller 6-2, 3-6, 4-6, 7-6 (7/5), 6-3 by the Spanish world number 67 on Court Suzanne Lenglen.
The 33-year-old has been plagued by a knee injury and arrived at Roland Garros having won just one match on clay in Geneva last week, his first event in three months.
The defeat means that the three-time Grand Slam champion will slip from 30th in the world to outside the top 250 after the tournament, having failed to defend the ranking points he gained in 2017.
"There is no frustration (at rankings slip). It's just tough," said Wawrinka, seeded 23rd this year.
"But again, I knew from the beginning that it will take a long, a lot of time to get back. I knew that from the surgery that it will take a year at least to get where I want to be."
There were worrying signs for his fans when he pulled up and called for a medical timeout after just six games, but he played down fears he had caused further damage to his already troublesome left knee.
"My knee is good. I blocked something else at the beginning of the first set, and that's why I had to ask," he said.
The 23rd seed found his rhythm from nowhere after a poor first set, crunching two massive forehands to break for a 3-1 lead in the second.
Djokovic eases through
Serbian star Djokovic had little trouble in reaching round two, though, seeing off Brazilian world number 134 Rogerio Dutra Silva 6-3, 6-4, 6-4.
The 12-time Grand Slam champion dropped serve three times and will have to markedly improve to challenge Nadal in the latter stages of the tournament, but he was still far too good for Dutra Silva on Court Philippe Chatrier.
"It was good to start this year with a win. I wasn't at my best, he played with a lot of spin. I didn't play very well, but I won in three sets," said the 20th seed, who will next face Spanish qualifier Jaume Munar.
Former world number one Victoria Azarenka's return to Grand Slam tennis lasted just two sets as the Belarusian slumped to a 7-5, 7-5 loss to Katerina Siniakova.
Azarenka, who missed much of last season after being embroiled in a custody battle over her son, was broken to love in the 12th games of each set to be dumped out by the Czech world number 57.
It was Azarenka's first Grand Slam match since losing to Simona Halep in the Wimbledon fourth round last July, but she still believes she can return to the top of the game.
"No doubt about it. Maybe not today, but no doubt about it," the now-world number 82 insisted.
Petra Kvitova also found life hard, but the two-time Wimbledon champion fought back from a set down to beat Paraguay's Veronica Cepede Royg 3-6, 6-1, 7-5 on Court Philippe Chatrier.
"I've won 12 matches straight on clay, but I'm still a long way from Rafa," she said in reference to Nadal's upcoming bid for an 11th men's singles crown.
Wozniacki held off the challenge of American Danielle Collins to ease through their match 7-6 (7/2) 6-1, while men's seventh seed Dominic Thiem, a semi-finalist in each of the last two years, brushed aside Ilya Ivashka in three sets.