Serena Williams made a slow but successful start Monday to her latest pursuit of a record-equalling 24th Grand Slam title at Roland Garros while US Open winner Dominic Thiem navigated a testing first-round tie against former major champion Marin Cilic.

The American star is chasing an elusive 24th Grand Slam title to match Margaret Court's all-time mark, but clay is her least favoured surface.

An error-prone beginning from Williams left world number 102 Kristie Ahn serving for the first set before the three-time Roland Garros champion prevailed 7-6 (7/2), 6-0.

Williams, playing at the French Open for the 18th time, dropped serve twice as Ahn moved 5-4 ahead in the opener in a rematch of their first-round encounter at Flushing Meadows.

But the sixth seed forced a tie-break that she sealed with an ace after a 74-minute slog before charging to an ultimately comfortable victory.

"She played very well in the first set, it wasn't easy for me and she hit a lot of winners," said Williams. "It was important for me to stay focused."

Williams, seeded sixth here, has not gone beyond the last 16 in Paris since her defeat to Garbine Muguruza in the 2016 final. Her last major came at the 2017 Australian Open while pregnant.

Meanwhile Thiem, the runner-up to Rafael Nadal in Paris the past two years, defeated 2014 US Open champion Cilic 6-4, 6-3, 6-3 in their second meeting at a major this month.

"My level was good. I was very happy with the way I played from the beginning to the end basically," said Thiem.

Having captured his maiden Slam in New York, Thiem is trying to become the only man in the Open era to win his first two majors at successive tournaments.

A steady display from Thiem saw the Austrian break Cilic six times under the roof of Court Philippe Chatrier on another gloomy day at Roland Garros, pushed back from its customary May-June slot due to the coronavirus pandemic.

"I really love this tournament, it's by far my best Grand Slam tournament so far," said Thiem, who plays American qualifier Jack Sock in the second round.

"I'm from Austria so I know how it is to play in these cold conditions. I love it when it's not too fast. I felt well from the first moment I came here."

Nadal faces 'most difficult' defence 

Nadal, on a mission to match Roger Federer's record of 20 major titles, arrives at the French Open short of preparation and unhappy with the tournament's choice of new balls.

"The conditions here are probably the most difficult conditions for me ever at Roland Garros for so many different facts," said Nadal, who won the first of his 12 French Open titles in 2005.

"The ball is completely different. It is super slow, heavy. It's also very cold, slow conditions."

Nadal has been beaten just twice in 95 matches at the tournament but now must adjust to the unusual autumnal chill in Paris. 

The heavier atmosphere could negate some of Nadal's lethal topspin as he heads into the event without a clay title under his belt for the first time.

However a first-time meeting with Egor Gerasimov, the world number 83 from Belarus, is unlikely to trouble him.

The competition began Sunday amid steady drizzle in temperatures struggling to reach 10 degrees (50F), conditions branded "ridiculous" by US Open finalist Victoria Azarenka.  

Play on Monday was initially delayed by more persistent rain. Twice Wimbledon champion Petra Kvitova defeated Oceane Dodin 6-3, 7-5 in the only match to start on time under the new roof on Court Philippe Chatrier. 

Dutch fifth seed Kiki Bertens recovered from a poor start once conditions improved to overhaul 112th-ranked Ukrainian Katarina Zavatska 2-6, 6-2, 6-0.

Third seed Elina Svitolina scrapped past Varvara Gracheva of Russia 7-6 (7/2), 6-4 while last year's runner-up Marketa Vondrousova was bundled out 6-1, 6-2 by 2018 junior champion Iga Swiatek.

Overall prize money at this year's event is down 11 percent at 14,491,000 euros, but players beaten in the first round will receive 60,000 euros -- a 30 percent increase compared to 2019.