Top-seeded Karolina Pliskova and No. 2 Sofia Kenin crashed out of the Western & Southern Open on Sunday, falling in their opening matches at the National Tennis Center in New York.

Russia's Veronika Kudermetova, ranked 41st in the world, toppled world number three Pliskova of Czech Republic 7-5, 6-4 in their second-round clash.

France's Alize Cornet notched her first win over a top-five player in two years when she shocked Australian Open champion Kenin 6-1, 7-6 (9/7) to reach the round of 16.

It was a stunning turn of events on a day that saw men's fourth seed Stefanos Tsitsipas lead the way into the round of 16 with a 6-1, 6-3 victory over two-time Grand Slam finalist Kevin Anderson.

Greece's Tsitsipas needed just 69 minutes to subdue Anderson, pressuring the towering South African's second serve as he broke four times.

He didn't face a break point himself until the fifth game of the second set, when Tsitsipas fell into a 0-40 hold but won five straight points to escape the jam.

"That was a crucial game, being able to come back there and maintain the focus that I needed to be back in the match and not be a break down, that was a very good effort from me," Tsitsipas said.

He gained the lone break of the second set when Anderson double-faulted on break point in the eighth game, and finished off the match on his second match point when Anderson sailed a return of a second serve long.

Tsitsipas was the first of the top eight men's seeds to see action as all enjoyed first-round byes.

World number one and top seed Novak Djokovic was scheduled to open his campaign on Monday against qualifier Ricardas Berankis.

The Serbian star remained on Monday's slate despite pulling out of doubles on Sunday citing pain in his neck.

US star Serena Williams, seeded third, is also set to open her campaign on Monday with a second-round clash with Arantxa Rus -- a 6-2, 6-3 winner over Alison Van Uytvanck.

Tsitsipas, seeking his sixth ATP title, next faces either 16th seeded American John Isner or Australian John Millman, both first-round winners on Sunday.

Isner beat his doubles partner Hubert Hurkacz of Poland 7-5, 6-4. Millman rallied to beat France's Adrian Mannarino 4-6, 6-4, 7-6 (7/2).

In other first-round action, Argentina's Diego Schwartzman defeated Casper Ruud 7-6 (7/2), 6-3 to advance.

While the tournament is being played without fans -- as well as without line judges with the electronic "hawkeye" line calling system making all the calls -- Schwartzman still found himself waving out of habit to the non-existent crowd after the win.

"Obviously it's a different feeling, going on court without people," Schwartzman said.

The tournament normally played in Cincinnati is being held at the US Tennis Association's Billie Jean King National Tennis Center, allowing players to remain in a controlled environment to prevent the spread of COVID-19.

The US Open will begin at the venue on August 31. Pliskova, the 2016 US Open runner-up, can only hope she can regroup by then.

Kudermetova, who laboured to a three-set first-round victory over Australian Ajla Tomljanovic on Saturday, rallied from a 1-4 deficit in the first set, and reeled off the last three games of the match to score the upset.

"I tried to not think about the score. I tried to fight, tried to continue play every ball," Kudermetova said. "I tried just to fight and enjoy it."

Pliskova had seemed in full control when she held at love for a 4-1 lead. The big-serving Czech peppered Kudermetova with 11 aces but still faced 13 break points in the match.

Praying for the best 

Cornet almost let her chance against Kenin get away. Up 6-1, 5-2, she squandered two match points before pulling off the upset in the tiebreaker.

"Somehow I let it go a little bit, and I got really tight, so I never thought I could win the set the way I did," said Cornet. "In the tiebreak, oh my God, I was just praying for the best on every single shot."

Kenin, the world number four who shot to prominence with her Australian Open win in January, next faces either two-time Australian Open champion Victoria Azarenka or French compatriot Caroline Garcia, who defeated 2017 US Open champion Sloane Stephens 6-3, 7-6 (7/4) in a first-round clash.