Novak Djokovic had to come from behind to beat Kei Nishikori and keep his bid for a calendar Grand Slam alive as he advanced to the US Open fourth round.

The Serb, 34, is aiming for a men's record 21st Grand Slam title.

He will face either Russian 21st seed Aslan Karatsev or American wildcard Jenson Brooksby next.

Top seed Djokovic has received a mixed reception from the New York crowd but appeared to have won them over by the final point.

He asked them to give him more noise several times, cupping his ear toward the fans, and did not do his traditional 'show of love' celebration to the stands.

He did, however, spend some time after his post-match interview signing autographs and giving several young fans items from his kit bag.

Djokovic looked out of sorts in the opening set, although he battled back from an early break down to force a tie-break.

However, he struggled with his serve, and Nishikori took full advantage to take just his fourth set off Djokovic out of the past 33 they have played.

Nishikori fought hard but was only able to convert two of the 13 break-point opportunities he created.

One of those breaks came as Djokovic served for a 5-3 lead in the third set, but the Serb instantly hit back and cruised through a routine fourth to close out the match.

"I don't think I started off very well but he surprised me with his level today," Djokovic said.

"It took me a little bit of time to adjust to his game."

German qualifier Oscar Otte reached the fourth round of a Grand Slam for the first time with victory over Italy's Andrea Seppi.

World number 114 Otte, who was match point down in his first two qualifying matches, won 6-3 6-4 2-6 7-5.

He will face Wimbledon finalist Matteo Berrettini, who had to work hard to see off Belarusian Ilya Ivashka.

The Italian sixth seed hit 69 winners as he came through 6-7 (5-7) 6-2 6-4 2-6 6-3.

Italian 13th seed Jannik Sinner overcame Gael Monfils in five sets, weathering a fightback from the Frenchman to win 7-6 (7-1) 6-2 4-6 4-6 6-4.