Once Agnieszka Radwanska was able to hold her serve, it got easy.

Radwanska and Zhang Shuai traded breaks Sunday in the opening seven games of their match, but the third-seeded Pole finally held and won the final nine games to become the first player to reach the second round at the French Open, winning 6-3, 6-0.

"So many breaks in a row," said Radwanska, who blamed her struggles on the wet court on an overcast day at Roland Garros. "I don't know if there is really any explanation why there was eight breaks in a row, but I guess it happens sometimes."

Radwanska reached the Wimbledon final in 2012, but her best result at Roland Garros was a spot in the quarterfinals last year.

"Clay is not really my favorite surface, but I'm trying," Radwanska said. "Last year I did my first quarterfinal, so hopefully this year I can do even better."

Radwanska started her match with a break to a take a 1-0 lead, but her serve failed her moments later when Shuai was able to even the score. The same happened over the next four games.

But after a fourth break, Radwanska finally held to make it 5-3.

"When I just broke myself in the end of the first set I think was more confident, and then I think I start to play much better," Radwanska said. 

In other Day 1 action Sunday, Serena Williams began the defense of her French Open title with a 6-2, 6-1 victory over 138th-ranked Alize Lim, a wild-card entry from France.

The men's action was headlined by fourth-seeded Roger Federer getting back to his winning ways.

The top-seeded Williams dictated most points and wound up with 30 more winners -- but also 24 more unforced errors -- than Lim.

Williams is a 17-time major champion who was facing a woman making her Grand Slam debut. Still, there was a bit of shakiness from the 32-year-old American as she served out the final game, erasing four break points and needing five match points to close things. Williams saved all seven break points she faced in the match.

She improved to 54-1 in first-round matches at majors. The only loss came at the 2012 French Open against 111th-ranked Virginie Razzano. 

Meanwhile, Venus Williams advanced to the second round by beating Belinda Bencic of Switzerland 6-4, 6-1.

Williams was down a break early in the first set, but she was able to recover and take control of the match.

The 29th-seeded American has won seven Grand Slam titles, but she lost to little sister Serena the only time she reached the French Open final in 2002.

This season, Venus won in Dubai in February, her first title since 2012. 

No. 8 Angelique Kerber of Germany, No. 14 Carla Suarez Navarro of Spain and 31 Daniela Hantuchova of Slovakia also advanced to the second round Sunday. No. 25 Kaia Kanepi of Estonia lost in straight sets to Monica Niculescu of Romania.