Two-time champion Novak Djokovic moved into the US Open third round after needing four sets to beat big-hitting American Tennys Sandgren.

Djokovic, 31, missed a match point in the third set before Sandgren dominated the tie-break.

But the Serb broke twice in the fourth set to win 6-1 6-3 6-7 (2-7) 6-2.

The sixth seed, chasing a 14th Grand Slam title, will play France’s Richard Gasquet or fellow Serb Laslo Djere next.

“I thought I played very well for the first two sets, two sets and a half, and then lost it mentally, said Djokovic, who praised Sandgren for his fight.

“I’m not happy with the way I lost concentration and composure but I managed to regroup in the fourth.”

Djokovic not at best but avoids major drama

Djokovic survived a slight scare in his opening match after needing “survival mode” in sweltering conditions on Tuesday lunchtime to battle past Hungarian opponent Marton Fucsovics.

In lower heat and humidity in Thursday’s night session on Arthur Ashe Stadium, there was no major drama for Djokovic – despite 27-year-old Sandgren making the former world number one stay out on court longer than he would have liked.

Sandgren, ranked 61st, won just six games when the pair met at Wimbledon in July and looked set for a similar pummelling.

Djokovic edged the key points in a first set which he won in 32 minutes, then was untroubled on his serve in the second set before breaking Sandgren twice to win the final four games.

But Sandgren, backed by a supportive home crowd under the floodlights, started finding his range in a third set where he hit 16 winners.

He saved a match point at 5-4 with a forehand down the line and served out again to force the tie-break.

Sandgren’s power blew Djokovic out of the breaker, but he ran out of steam as Djokovic wrapped up the fourth set in 33 minutes.

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Elsewhere, seventh seed and 2014 champion Marin Cilic raced through a 6-2 6-0 6-0 win over Poland’s Hubert Hurkacz in just one hour 19 minutes.

World number four Alexander Zverev advanced to the third round for the first time with a comprehensive win over France’s Nicolas Mahut.

The 21-year-old German won 6-4 6-4 6-2.

Fourteenth seed Fabio Fognini fell victim to Australian John Millman, losing 6-1 4-6 6-4 6-1.

Kei Nishikori progressed to the third round when Frenchman Gael Monfils retired injured in their match with the Japanese 21st seed leading 6-2 5-4.

Monfils was hurt when a volley hit his right wrist.