'Class act': US Open star's incredible gesture for injured rival

US Open fans were brought to their feet after an incredible moment of sportsmanship on day four at Flushing Meadows.

Perennial fan favourite Gael Monfils booked his place in the third round of the US Open with an absorbing 7-5, 4-6, 6-4, 6-4 over local hope, Steve Johnson.

WOW: Cameras catch Ash Barty in staggering act at US Open

'DEVASTATING NEWS': Tennis rocked by star's tragic death at 42

Monfils lit up the court in his fluorescent shorts and the flashy Frenchman proved once again why he's considered one of the great entertainers in men's tennis.

The 17th seed was at his flamboyant best in the four-set win but he suffered an early scare after falling awkwardly in the first set.

Monfils lay on the court for several moments at 3-3 in the opening stanza, clutching at his wrist as the crowd held its collective breath.

Without hesitation, Johnson stepped over the net and crossed the court to check on his stricken opponent.

The American had a quick chat to Monfils to make sure he was alright, before holding out a hand to help his rival up.

Johnson's beautiful gesture and concern for his opponent brought a rapturous round of applause from fans inside the stadium.

The wonderful moment also lit up social media, where viewers hailed the incredible act of sportsmanship on display.

Novak Djokovic's record quest alive

The four-set win for Monfils saw the Frenchman book a third round match-up against exciting Italian youngster Yannick Sinner, who also prevailed in four sets against another American, Zachary Svajda 6-3 7-6(7/2) 6-7(6/8) 6-4.

Joining them in the third round is World No.1 Novak Djokovic, whose bid for a record-breaking 21st major and calendar year Grand Slam gathered pace with a 6-2 6-3 6-2 thrashing of Tallon Griekspoor.

Having shaken off the rust in a spotty four-set opening match against 18-year-old Holger Rune on Tuesday, Djokovic was sharper and much more focused against the 25-year-old Dutchman.

Just like his first-round contest Djokovic dominated early on but this time the 34-year-old Serb never let his foot off the gas as he registered his 77th win at the US Open.

Only three men have won more matches at the US Open - Jimmy Connors (98), Roger Federer (89) and Andre Agassi (79).

Djokovic, playing his first event since the Tokyo Olympics, found the intensity that had been lacking against Rune, speeding through the opening set in 30 minutes.

"I came out on court with the right intensity, the right focus," said Djokovic. "Better definitely than the first-round match.

"It was important I came up with the goods.

"I'm very pleased with the level of my tennis. All is going in the right direction."

Afterwards Djokovic basked in the applause of the crowd, something he had not done for his opening round victory after the Arthur Ashe Stadium crowd had roared on his Danish opponent throughout the match.

After beating two young opponents playing in their first US Open main draw, Djokovic next faces Japan's Kei Nishikori, the 2014 US Open runner-up.

Earlier, Alexander Zverev thrashed Spain's Albert Ramos-Vinolas 6-1 6-0 6-3 in a brisk 74 minutes, conserving his energy as he rolled through to the third round.

with AAP

Click here to sign up to our newsletter for all the latest and breaking stories from Australia and around the world.