Serena Williams made a slow but successful start to her latest pursuit of a record-equalling 24th Grand Slam title at Roland Garros.
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 No 102 Kristie Ahn serving for the first set before the three-time Roland Garros champion prevailed 7-6, 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.
No player in the French Open women's draw has more claycourt wins than Kiki Bertens since the start of 2016 and the Dutchwoman needed all her expertise on the surface to avoid a first-round exit on Monday.
The fifth seed looked understandably rusty as she took on French Open debutant Katarina Zavatska, losing the first set before hitting back to win 2-6 6-2 6-0.
It was only her third match since February, having opted against travelling to the U.S. Open, and in Strasbourg last week she retired injured with an Achilles tendon strain.
But she gradually got up to speed against the 20-year-old Zavatska after being outplayed in the first set in which she saved seven set points.
Bertens, a semi-finalist here in 2016, cut out the errors in the second set and polished off the third in 31 minutes although her opponent was not helped by breaking three racket strings in the space of two games midway through the decider.
It will not get any easier for red-dirt specialist Bertens who will face former runner-up Sara Errani in the next round after the Italian qualifier thrashed Olympic champion Monica Puig in her first Grand Slam match for two years.
Errani reached the final in 2012 but in 2017 tested positive for a banned substance she claimed she had inadvertently ingested after her mother's cancer medication had got mixed up with a bowl of pasta. In 2018 the Court of Arbitration for Sport (CAS) increased her suspension by 10 months.
"It's going to be a really tough match," Bertens said, "I think I have to be ready to play a really long match, long rallies. I have to be patient."