The Serbian is looking to move clear of Margaret Court in the all-time list but arrived in New York having not played a hard court match since March.

He predicted the first couple of rounds would be testing for him and so it proved in his second-round clash against fellow Serbian Laslo Djere.

Djere, who took his compatriot to five sets in last year's tournament, gave him another battle and looked like winning the second set before a side injury caused him to retire with Djokovic leading 6-4 6-4 2-0.

Djere might have been inclined to carry on had he been able to win the second set.

He had two chances for a double break but Djokovic showcased his fighting spirit to turn it around and the end was nigh.

The 37-year-old knows he has to improve if he wants his hands on the trophy.

"It should have been his second set, he was 4-2 up at 15-40," Djokovic said. "I don't know if winning the second set put a mental burden on him.

"Overall it was a big fight, over two hours for two sets.

"I served awfully, playing without a serve you have to grind, you have to run.

"I'm just trying to push the bar for the young guys."

Elsewhere, American Frances Tiafoe got shirty after struggling with the heat.

Temperatures reached almost 40C in New York and the 20th seed was forced to change his shirt five times in 96 minutes in his clash with Alexander Shevchenko on Arthur Ashe.

The Kazakhstani also felt the heat as he retired trailing 6-4 6-1 1-0.

Tiafoe said: "They closed the roof on the court a little to give us some shade but it was still so hot.

"I have a pile of used shirts there Ã¢- I lost count of how many I changed during the match.

"It is so disgusting. I hate to win in that way but I was happy to get it done.

"I'm sweating crazy. What was it, 97 degrees outside? Today was crazy."

Sixth seed Andrey Rublev survived a scare as he fought back to beat Frenchman Arthur Rinderknech.

Rublev was two sets down but rallied to win 4-6 5-7 6-1 6-2 6-2.

Fourth seed Alexander Zverev had no such issues, cruising to victory 6-4 7-6 (5) 6-1 against Alexandre Muller.

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