The 38-year-old Serbian superstar made a winning start to his latest tilt at glory on Sunday, defeating US teenager Learner Tien 6-1, 7-6 (7/3), 6-2 in 2hr 25min on the Arthur Ashe Stadium court.

But Djokovic, who needed a lengthy medical timeout to treat a blister on his right foot between the second and third sets, admitted afterwards he was worried by a sudden loss of stamina during the second set.

"I started great -- just over 20 minutes, first set, I felt really good," Djokovic said.

"Then some long games to start the second set... I really was surprised how bad I was feeling in the second physically.

"We had long exchanges, but also, I kind of dropped my level and made a lot of unforced errors and kind of got him back into the match.

"There are positives but also things that hopefully won't happen in terms of how I felt on the court physically in the second set

"It's slightly a concern. I don't know. I don't have any injury or anything. I just struggled a lot to stay in long exchanges and recover after points."

- 'Still have the drive' -

Djokovic has not played since his defeat in the semi-finals at Wimbledon last month, skipping ATP Masters events in Toronto and Cincinnati to focus all his energy on the US Open.

While there were a few signs of rustiness at times on Sunday, Djokovic also laced his performance with flashes of the brilliance that has been his calling card throughout a glittering career.

Djokovic looked to be a man in a hurry in a scorching start, rattling off winners to take the first set 6-1.

Tien was given hope in a laboured second set from Djokovic, but the 19-year-old American could not convert a break point that would have made it one set all.

Instead, Djokovic held serve to make it 5-5 in the second set and reasserted himself to claim the second-set tie break, attacking the net cleverly and using some sublime variation to pull Tien all over the court.

Djokovic paused for a medical timeout for treatment to his right foot before the third set got under way.

The rest appeared to give Djokovic a jolt of energy and he sprinted into a 5-1 lead to leave Tien reeling.

Tien did well to save a match point on Djokovic's serve before breaking for 5-2.

But the respite was short-lived as Djokovic again responded emphatically, breaking back immediately to wrap up victory.

"I wish I had Learner Tien's age -- when you come to the late 30s it's about learning how to preserve the energy for what matters," Djokovic said.

"I still have the flair, I still have the drive, and you guys give me the energy. Hopefully I can keep it going," he told the crowd.

Djokovic will face US qualifier Zachary Svajda in the second round.