Four-time U.S. Open champion Novak Djokovic advanced to the third round in New York on Wednesday when his opponent retired in the third set.

Djokovic, seeded second, led 6-4, 6-4, 2-0 when his Serbian countryman, Laslo Djere, was forced to retire due to an apparent abdominal injury.

Djere led 4-2 in the second set and had two break points before losing the next four games. He took a medical timeout late in the second set, and he didn't win any of the eight points contested in the third set before shutting it down.

"It's not what we want," Djokovic said. "It's not what the crowd wants, to see a walkover. But he obviously had an injury that took him out of the tour for some time and he is struggling to come back physically to this level. He's such a good player, especially in these conditions. It should've been his second set, honestly. It was 4-2, 15-40.

"Overall, it was a big fight. Over two hours for two sets. I served awful. So playing without the serve, you have to grind, you have to run. So I guess I had to rely on my baseline game."

Next up for Djokovic will be No. 28 Alexei Popyrin of Australia, who swept Spain's Pedro Martinez 6-2, 6-4, 6-0.

Fourth-seeded Alexander Zverev unleashed 15 aces while recording a 6-4, 7-6 (5), 6-1 victory over Alexandre Muller of France.

Zverev held a 43-27 edge in winners while beating Muller, who committed 41 errors. Zverev had 29 miscues in the intense heat.

"I thought he played some amazing tennis. Especially at the end of the second set, I thought he was the better player than me," Zverev said of Muller.

"But I found a way and of course I am happy to be done in three sets. To have some rest as it is very hot and tough conditions."

Zverev will next face Tomas Martin Etcheverry, who posted a 6-3, 4-6, 6-4, 1-6, 6-3 triumph over fellow Argentine Francisco Cerundolo, the No. 29 seed.

Sixth-seeded Andrey Rublev of Russia recovered from a two-set deficit to notch a 4-6, 5-7, 6-1, 6-2, 6-2 win over Frenchman Arthur Rinderknech. Rublev next faces No. 32 Jiri Lehecka of the Czech Republic, who also staged a frantic comeback during a 6-7 (5), 0-6, 6-4, 6-4, 7-5 victory over Mitchell Krueger.

Eighth-seeded Christian Ruud of Norway topped France's Gael Monfils 6-4, 6-2, 2-6, 7-6 (3), and No. 12 Taylor Fritz of the United States eliminated Italy's Matteo Berrettini 6-3, 7-6 (1), 6-1.

No. 9 Grigor Dimitrov of Bulgaria knocked off Australia's Rinky Hijikata 6-1, 6-1, 7-6 (4). He next faces Tallon Griekspoor of the Netherlands. Griekspoor led 6-1, 2-0 when Argentina's Sebastian Baez retired due to a right foot injury.

No. 13 Ben Shelton smashed 17 aces while sailing to a 6-3, 6-4, 6-4 victory over Spain's Roberto Bautista Agut. Shelton will face Tiafoe in the third round.

No. 18 Lorenzo Musetti of Italy pulled out a 3-6, 6-4, 6-4, 2-6, 7-5 win over Serbia's Miomir Kecmanovic.

No. 20 Frances Tiafoe of the United States moved on when Alexander Shevchenko of Kazakhstan retired early in the third set due to a knee injury. Tiafoe won the first two sets 6-4, 6-1 and took the first game of the third set before Shevchenko called it a day.

"Happy to get it done," Tiafoe said. "Hate to win it that way but overall I thought I actually played pretty well today."

No. 17 Ugo Humbert of France won the opening set before falling 5-7, 6-4, 6-4, 6-4 to Argentina's Francisco Comesana.

Also, Brandon Nakashima cruised to a 6-4, 6-4, 6-2 victory over Arthur Cazaux, and China's Juncheng Shang beat Spain's Roberto Carballes Baena 6-2, 6-3, 7-6 (2).

--Field Level Media