Novak Djokovic hopes the "physical struggle" he faced before powering to victory in the US Open first round was simply down to a lack of match practice.

The 38-year-old Serb, who earned a 6-1 7-6 (7-3) 6-2 win over American teenager Learner Tien, had not played since losing in the Wimbledon semi-finals in early July.

During the second set Djokovic's level dropped considerably as he looked increasingly flustered and unsteady on his feet in a humid New York.

The 24-time major champion needed treatment for blisters on his toe after securing the second-set tie-break, but said the issue was not the reason behind his dip.

"I don't have any injury or anything. I just struggled a lot to stay in long exchanges and recover after points," said Djokovic, who is seeded seventh.

"I haven't played an official match in six weeks. I had to deliver my A-game, which I think I did in the first set.

"In the second, particularly, it was surviving on the court. In the important moments I just put one ball more in the court than he did."

As Djokovic heads towards retirement, he has continued to scale back his schedule.

The former number one's focus has been increasingly centred on the Grand Slam tournaments as he aims to land a standalone record 25th major title.

Djokovic skipped the ATP Masters events in Toronto and Cincinnati leading up to the US Open.

In his pre-tournament news conference, Djokovic made a frank admission that he does not "enjoy" the elongated format of the Masters tournaments, which have been stretched to two weeks over recent seasons.

But when it comes to the biggest prizes, his appetite to win Grand Slam tournaments remains undimmed.

Beating the precociously-talented Tien, who is ranked 50th in the world, was Djokovic's 75th consecutive first-round win at a Grand Slam event - the first singles player to achieve the feat in the Open era to achieve the feat.

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

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

American pair Ben Shelton and Taylor Fritz cruised into the second round as they began their latest bids to end the USA's 22-year wait for a Grand Slam men's singles champion.

Andy Roddick was the last American man to win a major singles title with his 2003 US Open triumph.

Fritz, 27, has come closest to breaking the drought, having reached the final at Flushing Meadows last year - making him the first American man in 15 years to reach a major showpiece.

Seeded fourth this year, Fritz beat compatriot Emilio Nava 7-5 6-2 6-3, while sixth seed Shelton dispatched Grand Slam debutant Ignacio Buse 6-3 6-2 6-4.

It is the first time since the 2007 US Open that two American men have been seeded inside the top eight.

"We're in a great place in American tennis," Fritz said.

"The women have been really carrying for a long time, but the men - we're coming."

Shelton, 22, was the first player of this year's tournament to play on the iconic Arthur Ashe show court, taking just over two hours to wrap up victory.

The former semi-finalist is in good form, having won his first ATP Masters 1000 title at the Canadian Open earlier this month.

"The US Open is the pinnacle of tennis for me and what I dreamed about as a kid," Shelton said.

"It is starting to feel like home for me."

Fritz will face either Sebastian Beez or qualifier Lloyd Harris in the second round, whilst Shelton will take on Spain's Pablo Carreno-Busta.

Teenage sensation Jakub Mensik had to work hard for a 7-6 (7-5) 6-3 6-4 victory over Chile's Nicolas Jarry as play got under way on Sunday.

Mensik, seeded 16th, won his first ATP Tour title back in March, beating Djokovic in the Miami Open final, and has twice reached the third round in New York.

The 19-year-old Czech will face Ugo Blanchet next after the French qualifier beat Hungary's Fabian Marozsan 6-4 3-6 7-6 (9-7) 6-2.

Elsewhere, Spanish 18th seed Alejandro Davidovich Fokina needed only 89 minutes to beat Alexander Shevchenko 6-1 6-1 6-2.

Dutch player Tallon Griekspoor was the only men's seed to fall on the opening day, losing 7-5 6-4 6-0 to French veteran Adrian Mannarino.