Images from the US Open men's singles second round matches, at Flushing Meadows, New York, on Wednesday.

Djokovic advances to third round after Djere retires injured

Defending US Open champion Novak Djokovic kept his bid for a record 25th Grand Slam title on track on Wednesday after fellow Serb Laslo Djere retired injured while trailing 6-4, 6-4, 2-0 in their second-round match.

Djokovic had needed five sets to get past Djere in the third round a year ago and it looked as though Arthur Ashe Stadium would see another epic clash between the two as Djere went up a break in the second set.

But the unseeded Serb appeared to pick up an injury midway through the set and Djokovic was gifted his 90th win at the U.S. Open, making him the first man to reach that figure at each of the four Grand Slams.

Four-time champion Djokovic, who had looked rusty in his opening round match, appeared to struggle in the sweltering conditions that tested competitors all day at Flushing Meadows, wrapping himself in ice towels on the changeovers.

Djokovic broke Djere to love in the 10th game of a tightly fought first set, benefiting from a series of unforced errors by his opponent.

The second seed had to work harder to hold serve in the opening game of the next set, fending off two break points, and then shouted in frustration as he let a break point opportunity slip through his fingers in the next game.

Djere looked to have the momentum as he broke his opponent in the third game of the second set and held serve in the next with a mighty ace.

But the evening fell apart for Djere when he winced after a serve in the eighth game and Djokovic converted a break point.

Djere, who missed part of the season after undergoing elbow surgery in April, appeared to struggle with his mobility from there and took a medical timeout after the ninth game, lying on the court as a physio massaged his abdomen and lower back.

Djokovic broke again in the 10th and Djere tried to fight on before retiring seven minutes into the third set.

Up next for Djokovic is Australian Alexei Popyrin, who took a set off him in their previous Grand Slam clashes at Wimbledon and the Australian Open this year.

Zverev blasts past Muller to reach third round

Germany's Alexander Zverev fought through to the US Open's third round on Wednesday, dispatching Alexandre Muller 6-4, 7-6(5), 6-1 on a sweltering Louis Armstrong Stadium court.

The big-serving fourth seed blasted 15 aces and employed a drop-shot finesse at the net to preserve his hopes of capturing a career-first major after a runner-up finish at Roland Garros.

Zverev proved too much for world No. 77 Muller, who committed 41 unforced errors and battled through an apparent leg injury in brutal temperatures and high humidity at Flushing Meadows.

The German came out firing after a shaky round-one performance, breaking Muller's serve in the first game before ultimately claiming the back-and-forth first set.

Clutching his upper right leg, Muller left the court for a lengthy medical time-out between sets but returned with a vengeance.

The resilient Frenchman stood two points from taking the second set but the world No. 4 fought back to take the final five points of a tiebreak, blasting a line-hugging backhand for the winner.

Zverev cruised through the third set, closing out the match with a final ace.

"(I played) much better than in the first round. I thought at some point (Muller) also played some very good tennis, especially in the second set," Zverev said.

"The second set tie-breaker was high-level. I was down 5-2 and him serving but I didn't feel like I did much wrong. I'm happy to have turned that around and then ran away with it in the third set."

The win sets up a clash in the next round with Argentina's Tomas Martin Etcheverry, whom Zverev beat in four sets at Roland Garros last year but who played a four-hour, five-set match in brutal heat on Wednesday.

"I'm happy to be through in three sets because I think weather and conditions were obviously very difficult today. Very hot, very humid," Zverev said.

"The less time I spend on the court, the better it is." 

Rublev fights back from two sets down to beat Rinderknech

A tenacious Andrey Rublev survived a marathon encounter, battling back from two sets down to beat Arthur Rinderknech 4-6, 5-7, 6-1, 6-2, 6-2 and move into the third round.

Few would have expected Frenchman Rinderknech to trouble four-times US Open quarter-finalist Rublev, who had a 2-0 winning record against the world number 56, with one of those wins coming in this tournament last year.

The Russian took his time to get going in sweltering conditions on Court 17, and was put under serious pressure early on by Rinderknech as he broke to take a 2-1 lead in the first set.

Rublev, who has struggled to keep his temper under control at times on tour, fought back to level the set at 4-4 but his serve was broken in the next game and he repeatedly smashed his racket into the ground in frustration.

In a gruelling second set that lasted more than 80 minutes, Rublev edged ahead 3-1 but Rinderknech broke back in a game that featured a remarkable 37-shot rally which left the Frenchman lying flat on his back and gasping for air.

Rinderknech went on to clinch the second set but the effort appeared to take a toll on him, and his performance tailed off significantly in contrast to the opening two sets, where he fired 37 winners compared to just 18 in the final three sets.

Sixth seed Rublev gleefully took advantage and tapped into all of his experience to secure victory with four hours and six minutes on the clock, setting up a meeting with Jiri Lehecka of the Czech Republic.

Tiafoe in third round after Shevchenko retires

Big-hitting Frances Tiafoe boost American hopes at the Open by booking a ticket into the third round in convincing fashion.