San Francisco Frontiers - Three matches to watch on Friday at the US Open

Djokovic, chasing a fifth US Open title and a record 25th Grand Slam crown, said he "served awful" in a tight second-round victory over compatriot Laslo Djere, which ended when Djere withdrew while trailing early in the third set.

He'll have to do better against Australian Alexei Popyrin, who won the biggest title of his career this month when he triumphed at the ATP Montreal Masters.

Gauff, who coughed up nine double faults and put just 44 percent of her first serves in play in an uneven victory over Tatjana Maria, can expect a stronger challenge from 27th-seeded Ukrainian Elina Svitolina.

Second-seeded Aryna Sabalenka of Belarus, runner-up to Gauff last year and winner of the Australian Open in January, is also in action.

AFP Sport looks at three matches to watch on day five of 2024's final Grand Slam, which will conclude on September 8 (x denotes seeded player):

Alexei Popyrin (AUS x28) v Novak Djokovic (SRB x2)

Djokovic is 3-0 against Popyrin, including four-set victories at the Australian Open and Wimbledon this year. But Popyrin is riding the high of his biggest win at the Montreal Masters this month and Djokovic said the 25-year-old is "getting closer and closer and knocking on the door."

"He's pumped, he's confident and he shows that," said Djokovic, who claimed a long-coveted Olympic gold medal in Paris but is facing the prospect of a year without a Grand Slam for the first time since 2017.

"I know what to expect," Djokovic said. "I'm going to do my homework, analyze those matches, and see what needs to be done," he said.

But Popyrin has learned from their prior matches, too.

"I had chances in both matches and it's just a matter of taking those chances and playing good on the big points," he said.

Elina Svitolina (UKR x27) v Coco Gauff (USA x3)

Gauff and Svitolina have split their two career matches, but Svitolina's victory came back in 2021 at the Australian Open, when Gauff was just 16.

This year the American beat Svitolina in the WTA Auckland final in January to retain her title, but that's the only trophy she has lifted in what has been an erratic year.

"That was a tough, tough match," Gauff said of their Auckland clash. "We had a lot of physical points."

Svitolina, owner of 17 WTA titles, returned from maternity leave in April of last year and reached the quarter-finals of the French Open and semi-finals at Wimbledon. She reached the last eight at Wimbledon again this year. Her best US Open showing was a semi-final run in 2019.

"I think the challenge will be just finding the balance between being aggressive and not letting her dictate, but also not making too many errors," Gauff said.

Ben Shelton (USA x13) v Frances Tiafoe (USA x20)

Shelton advised tennis fans to get their popcorn ready to watch this one, a rematch of a memorable all-American quarter-final last year in which Shelton eliminated 2022 semi-finalist Tiafoe in straight sets.

"Being able to do it a little bit earlier in this draw, it still feels the same," 21-year-old left-hander Shelton said. "Still feels like a huge match."

Shelton has won both of their prior encounters, most recently in the final of the clay court ATP event in Houston in April.

But Tiafoe has won 11 of his last 15 matches after a coaching change and reached his first Masters 1000 final in Cincinnati this month.

Shelton said he thought the New York crowd would likely lean toward Tiafoe.

"He's electric here, and his crowds are electric here," Shelton said, predicting "a war, just like the last two times we played."

Y.Gomez--SFF