NEW YORK -- Karolina Pliskova can still win the U.S. Open and keep her No. 1 ranking, thanks to a big comeback Saturday.

Pliskova fought off a match point in the second set and rallied to beat No. 27 Zhang Shuai of China 3-6, 7-5, 6-4 to advance to the fourth round.

The runner-up last year needs to at least return to the final for a chance to remain atop the rankings when the year's final major tournament is over, and it appeared for a while that the Czech wouldn't get close.

But she stayed alive while trailing 4-5 in the second set, then pulled out the third after receiving treatment from a trainer on her right forearm between sets.

"Last Grand Slam of the season and knowing you played fine last year, you just want to play that good again," Pliskova said. "I don't feel like I'm playing that good again, but I'm still in the draw, actually, so I'm not going to be that sad."

Wimbledon champion Garbine Muguruza and No. 4 seed Elina Svitolina are the other women who can still get to No. 1. Svitolina was in action later Saturday against American Shelby Rogers, who won the longest women's match in U.S. Open history on Thursday when she beat No. 25 Daria Gavrilova in 3 hours, 33 minutes.

Pliskova, who fell to Angelique Kerber in the 2016 final, would need to win the tournament if Muguruza reaches the semifinals and loses. If Muguruza advances to the final, Pliskova would be eliminated from contention for the WTA's top ranking.

"It's not only about the points, but I just felt I really can play well here," Pliskova said.

Upsets have scrambled one half of the men's bracket, but everything is mostly as expected on the other side.

Roger Federer and Rafael Nadal were trying to keep it that way Saturday.

The two favourites are back in action with third-round matches in Arthur Ashe Stadium, where seeded Americans Madison Keys and CoCo Vandeweghe also were on the schedule.

Federer faces No. 31 Feliciano Lopez in a night match. The five-time U.S. Open champion has won all 12 meetings but hasn't played his top tennis yet in this tournament. He's played consecutive five-setters to open a major tournament for the first time.

Top-ranked Nadal plays Argentina's Leonardo Mayer.

Nadal, Federer, No. 6 Dominic Thiem and No. 9 David Goffin remain alive in their half of the bracket. No. 12 Pablo Carreno Busta is the top seed in the other, which will send a first-time Grand Slam finalist to Arthur Ashe Stadium for the championship match next Sunday.

Thiem eased into the fourth round with a 7-5, 6-3, 6-4 victory over No. 30 Adrian Mannarino of France. He will next face No. 24 seed Juan Martin del Potro. The 2009 champion swept past No. 11 Roberto Bautista Agut, 6-3, 6-3, 6-4.

Goffin was facing No. 18 Gael Monfils of France.

Also advancing were No. 33 seed Philipp Kohlschreiber, a straight-sets winner over Australian John Millman, and Alexandr Dolgopolov, who routed Viktor Troicki 6-1, 6-0, 6-4.

Dolgopolov is into the fourth round for the first time since 2011 and would be the next opponent for Nadal, but was peppered in his post-match news conference about questions related to a match he was involved in recently in Winston-Salem, North Carolina, that's under scrutiny because of unusual betting patterns.

The match is being assessed but is not yet under formal investigation, Tennis Integrity Unit spokesman Mark Harrison said earlier this week.

"Obviously it's disappointing, but not more," Dolgopolov said. "If people want to write something, they write something. You can't stop them from doing it. It's just not under my control."

French Open women's champion Jelena Ostapenko was ousted with a 6-3, 6-2 loss to Russia's Daria Kasatkina