NEW YORK — For the second year in a row, four women will split the year’s four Grand Slam titles.

Wimbledon champion Angelique Kerber fell, 3-6, 6-3, 6-3, to Slovakia’s Dominika Cibulkova on Saturday at the U.S. Open, joining Australian Open champion Caroline Wozniacki and French Open champion Simona Halep on the list of women upset at Louis Armstrong Stadium.

Kerber defeated Serena Williams in straight sets in July to claim her first Wimbledon title and third major overall but has struggled to gain any traction in the North American hard court season. Her season had been impressive beyond the Grand Slams; the German had made it at least to the quarterfinals in every tournament she played in 2018 before August. Kerber, the No. 4 seed in New York, played in the semifinals at the Australian Open and the quarterfinals at the French Open before winning in England.

But at hard court events in Montreal and Cincinnati she ran into players on hot streaks and fell early in both tournaments, arriving at the U.S. Open with five matches under her belt.

“I was trying to do the best I can, I mean, it was maybe not the best preparation I had before the U.S. Open,” said Kerber, the champion here in 2016 . “I didn’t really have a lot of matches on hard court. . . . You sometimes have days where you are trying everything, but it’s not really working out as you wish. I was trying everything until the last point.”

Cibulkova was the more aggressive player Saturday, and her reward is a matchup against American Madison Keys, runner-up here last year, in the round of 16. The No. 14 seed defeated Aleksandra Krunic, 4-6, 6-1, 6-2, at Arthur Ashe Stadium, which wasn’t immune to the upset bug Saturday.

No. 22 seed Maria Sharapova dropped No. 10 seed Jelena Ostapenko, 6-3, 6-2, in a mere 82 minutes on the tournament’s main stage at night.

“I feel like I started with that energy today,” said Sharapova, the 2006 U.S. Open champion. “She’s a phenomenal player, a Grand Slam champion, and I can’t come out here and expect an easy match. I think that’s what got me going, that competitiveness.”

Keys, despite her lower seeding that followed a dip in rankings after an injury-plagued 2018, is playing confidently enough to make another deep run in New York. She hit cruise control against Krunic after struggling with first-set jitters that caused her to badly whiff an overhead.

Her comfort in front of the big crowds on the tournament’s main stage helped her keep her head.

“Just the experience I think is a big help. I think the confidence comes more from knowing I was able to get myself back in so many matches last year after being down and, you know, even in final sets being down,” Keys said.

Two-time Wimbledon champion Petra Kvitova was the third high seed to fall at Armstrong Stadium on Saturday. The fifth-seeded Czech lost, 7-5, 6-1, to Aryna Sabalenka of Belarus.

Defending champion Sloane Stephens, the third seed, is the last of the top five seeded women remaining in the draw.

No. 4 seed Zverev ousted

The men’s draw saw its own big upset when No. 4 seed Alexander Zverev followed Kerber out the door at Louis Armstrong Stadium, falling, 6-7 (7-1), 6-4, 6-1, 6-3, to Philipp Kohlschreiber. Of the three Germans to play on the tournament’s No. 2 court Saturday, the 34-year-old Kohl­schreiber was the last one standing. He faces Kei Nishikori in the round of 16.

Zverev, meanwhile, will continue to search for the Grand Slam success that has eluded him. The 21-year-old’s record outside of majors in 2018 is 35-10. At Grand Slams, he is 10-4.