PARIS -- Venus Williams has reached the French Open's fourth round by beating an opponent who is 15 years younger than she is.

Williams, who turns 37 this month, did not get much of a test during a 6-3, 6-1 victory over 60th-ranked Elise Mertens of Belgium that lasted just over an hour at Court Philippe Chatrier.

The 21-year-old Mertens was playing in only her second Grand Slam tournament.

Williams is a seven-time major champion and was the runner-up at Roland Garros in 2002.

"Today I had experience," Williams told the crowd afterward, "and that helped a lot."

This is her record 20th appearance in the French Open.

"I'll be honest: There won't be 20 more," Williams said with a laugh. "I will be watching in 20 years with all of you guys."

Former No. 1 Caroline Wozniacki's third-round match against 18-year-old CiCi Bellis of California was suspended because of darkness.

Wozniacki took the first set 6-2, but Bellis was leading 5-2 in the second and about to serve for it when play was halted Friday night.

When Wozniacki was down 3-2 in the second set, with rain falling, she complained to the chair umpire that it was too dark to continue.

"I literally cannot see," Wozniacki said.

She resumed her plea to stop after dropping the first point of the next game, but was told by the chair umpire to continue. Wozniacki then walked over to the side of the court to talk to an official sitting in the stands.

They played on before eventually stopping. The match will resume Saturday.

Also on Friday, defending champion Garbine Muguruza advanced to the fourth round by beating Yulia Putintseva 7-5, 6-2.

The fourth-seeded Spaniard broke her opponent's serve three times in the second set.

Muguruza will next face 13th-seeded Kristina Mladenovic of France. Mladenovic beat American Shelby Rogers 7-5, 4-6, 8-6 from 5-3 down in the last set.

Former champion Svetlana Kuznetsova reached the fourth round after a grueling match lasting more than three hours.

The eighth-seeded Russian beat No. 32 Zhang Shuai of China 7-6 (5), 4-6, 7-5, improving to 5-1 in their head-to-head matches.

Kuznetsova wasted two match points serving at 5-3 and 40-15, and then dropped her serve. Zhang let out a loud scream when she held to love for 5-5.

But after holding her next serve, Kuznetsova took her chance on her third match point when Zhang's scooped shot from the back of the court sailed long.

No. 23 Samantha Stosur defeated American qualifier Bethanie Mattek-Sands 6-2, 6-2.