The US Open enters its fourth round on Sunday, with American hopes Taylor Fritz and Taylor Townsend facing tough tests as Novak Djokovic battles through injuries to continue his quest for a record 25th slam.

Top men's match: Tomas Machac V Taylor Fritz

Last year's finalist Taylor Fritz remains the home nation's best hope of becoming the first American man since 2003 to win the US Open title following the exits of sixth seed Ben Shelton and 17th seed Frances Tiafoe in the third round.

"Yeah, it sucks. I feel bad for those guys, especially I feel bad for Ben... it's a rough day for the guys," Fritz said referring to Shelton's retirement through injury.

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Fritz's own road to the fourth round has been rocky, as he dropped a set in his last two matches against Lloyd Harris and Jerome Kym, with the latter taking him to tiebreaker twice.

Tomas Machac, who plays Fritz on Sunday, has lost both of his previous meetings against the American, including a five-set tussle at the French Open in 2020.

The Czech 21st seed, who won his first ATP title in March, is yet to drop a set in New York.

Top women's match: Barbora Krejcikova V Taylor Townsend

Unseeded American Taylor Townsend is enjoying one of the best Grand Slam campaigns of her career after beating fifth seed Mirra Andreeva to reach the singles fourth round of a Grand Slam for only the second time in her career.

Townsend's campaign showed promise when she knocked out 2017 French Open winner Jelena Ostapenko in the second round. The 29-year-old's assured performance has made her one of the players to watch in the tournament.

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"As a tennis player, people have always said, oh, you're so talented... but, there was always a "but," Townsend told reporters. "I feel like the work I've been put in... has eliminated that 'but.' I'm emerging as a new person. I feel like today I levelled up."

Townsend, bidding to reach the quarter-finals of a major for the first time, will take on twice Grand Slam champion Barbora Krejcikova.

Krejcikova beat Townsend in their only previous meeting in 2017.

Battle-worn Djokovic driven by love for the game

Novak Djokovic dealt with a right foot blister in the first round, and the 38-year-old had to seek medical treatment again during his third-round clash for a back issue.

"I was kind of moving in the right direction... just made a quick move and reaction, and I felt the left side of my back," Djokovic said.

But the Serbian persisted to beat Cameron Norrie, setting up a last 16 meeting with Germany's Jan-Lennard Struff.

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Djokovic has beaten Struff in all seven of their previous meetings. But the seventh seed will also be battling the physical toll the demanding Grand Slam campaign is taking on him.

"The wear and tear on the body all these years is taking a toll, and I'm aware of it, but I'm resisting it. I'm trying to do my best to still be out there competing with the young guys in the highest level," Djokovic said.

"I'm asking myself questions... like, how long do I want to keep going at this level, and how do I want to approach my schedule to kind of extend my career, because I really want to play."