Serena Williams left frustrated in US Open warm-up while Novak Djokovic eases through

Missed opportunity for 23-time Grand Slam winner as she loses to Sakkari at the Western and Southern Open, as men's No 1 defeats Sandgren

Serena Williams was beaten in three sets by Maria Sakkari at the Western and Southern Open in New York on Tuesday, August 25. AP

Serena Williams likened the frustration she felt in Tuesday's shock defeat by Maria Sakkari at the Western and Southern Open to that of being stuck in a bad relationship.

Williams' 5-7, 7-6, 6-1 loss in the round of 16 was a big blow to her preparations for next week's US Open, where the 38-year-old is bidding for a record-equalling 24th Grand Slam singles title.

It was a second exhausting three-setter in as many rounds for Williams, who squandered the opportunity to close out the contest on serve in the second set before her Greek opponent launched a comeback.

"It was tough. I literally should have won that match. There was no excuse," Williams, who at one point flung her racket into the empty stands and also received a time violation, told reporters.

"I literally put myself in this situation. You know, it's like dating a guy that you know sucks. That's literally what I keep doing out here. It's like I have got to get rid of this guy. It just makes no sense. It's frustrating."

"It was hard ... I had so many opportunities to win and I have to figure that one out, like how to start winning those matches again."

Williams, whose match fitness had been questioned ahead of the Open, said she was also struggling with cramps towards the end of the match.

"I started cramping but shouldn't have been in that situation," she said. "I don't think that helps mentally when it's like, you know the match is over and you have won, and now your legs were already tired and now they're even more tired."

On the men's side, world No 1 Novak Djokovic won his 20th straight match of 2020 with a hard-fought 6-2, 6-4 victory over Tennys Sandgren.

Djokovic, who captured the Australian Open in February, needed six match points to tough out the straight sets victory and reach the quarter-finals of the hardcourt tournament.

Djokovic breezed through the final game by winning four straight points, closing it out with a cross-court forehand winner to take it in 88 minutes on Tuesday.

"All in all it was a great performance," said Djokovic. "I felt better and played better than last night. I am going in a great direction."

Novak Djokovic defeated Tennys Sandgren 6-2, 6-4 at the Western and Southern Open. USA TODAY Sports

American Sandgren, who is ranked 55th in the world, made the Serb work for the victory as he survived five match points in the ninth game of the second set.

Djokovic advances to the quarters where he will play German Jan-Lennard Struff, who defeated seventh seed David Goffin 6-4, 3-6, 6-4.

Djokovic had pulled out of doubles on Sunday with pain in his neck, but did not appear to be bothered by the injury against Sandgren. "The neck is doing well," he said.

Also on the men's side, hard-serving Canadian Milos Raonic routed Brit Andy Murray 6-2, 6-2 and third seeded Daniil Medvedev, of Russia, swept past Aljaz Bedene 6-3, 6-3.

In other women's action, fourth seeded Naomi Osaka rolled over Dayana Yastremska in straight sets 6-3, 6-1 to advance.

Osaka, who is the only top 10 player left in the women's draw, clinched the victory when Yastremska was called for a foot fault while serving on match point.