Novak Djokovic continued his bid for a record 25th Grand Slam title in style as he stormed past qualifier Jan-Lennard Struff to reach the US Open quarter-finals.

Despite twice receiving medical attention, the Serb rarely looked troubled as he raced to a dominant 6-3, 6-3, 6-2 victory in under two hours.

While the win was further evidence of Djokovic's pedigree, questions remain over the 38-year-old's physical condition.

Djokovic is playing his first tournament since losing in the Wimbledon semi-finals in July, having skipped ATP Masters events in Toronto and Cincinnati.

He looked fatigued in his opening two matches in New York, while he struggled with a lower back injury in his third-round win over Britain's Cameron Norrie.

Again, there were signs of wear and tear as he called for the physio to treat a neck issue and then a problem with his right forearm against German Struff.

However, the setbacks did not appear to harm his tennis as he dictated play with relentless hitting - firing 33 winners and 12 aces, with no double faults.

The world number seven highlighted his serve to keep him out of trouble on Arthur Ashe Stadium.

"It definitely helps if you serve well. I think I had a great serving performance in the last round and also tonight," Djokovic said.

"I just saw the stats, I out-served one of the guys that had the most aces during the tournament this year, so that's a great stat.

"Obviously, that helps make my life easier on the court. Maybe I don't need to work as hard or unnecessarily more than I have to."

Djokovic will face American home hope Taylor Fritz in the last eight after the fourth seed claimed a commanding straight-set win over Czech Tomas Machac.

Djokovic wants quarter-final win for daughter's birthday present

Speaking about his upcoming quarter-final, Djokovic said his daughter Tara was displeased it will take place on the same day as her birthday.

"She was not very happy about it, me being absent from the birthday party. Don't remind me of that, please," said Djokovic, who dedicates his violin celebration to his daughter at the end of each win.

"I'll try to win if I'm here. At least I'm going to try to win and give her that kind of present.

"I'm going to send some nice presents, as well, nice surprises for her birthday party."

At 38 years and 94 days old, Djokovic became the oldest player in the Open era to reach the quarter-finals at all four Grand Slam events in a single season.

Asked about playing the night session on Arthur Ashe Stadium, Djokovic said: "I don't know how many more I am going to have so each one is very special."

With time ticking down on his illustrious career, there is less room for error in his pursuit of a standalone record 25th major title.

He got off to the best of starts against Struff, racing to a 4-0 lead with a double break before his opponent made a first impression on the scoreboard.

Djokovic's momentum slowed slightly as he seemed to struggle with his neck and he called for the physio immediately after coming through the opening set 6-3.

However, Djokovic carried on without real problems and, after a sluggish start to the second set, he won four games in a row to assert a two-set lead.

Once again, the four-time US Open winner received treatment at the end of the set - this time for a problem with his right forearm.

Whatever was bothering Djokovic appeared to have little impact on his play. After breaking early in the third set, he saw out the win with confidence to reach his 64th Grand Slam quarter-final in his 80th appearance.