A US Open full of upsets and hungry, young contenders offered the latest sign that men's tennis has entered a new era, with 2024 marking the first time in more than two decades that none of the "Big Three" claimed a Grand Slam title.

With Roger Federer retired and Rafael Nadal struggling to get back on court due to injuries, Novak Djokovic was the last man of that golden generation standing at Flushing Meadows, but the Serb crashed out in the third round to Australian Alexei Popyrin.

While Djokovic's wait for a record 25th Grand Slam goes on, Jannik Sinner picked up his second major of the year with his victory over Taylor Fritz in Sunday's final.

The Italian also won the Australian Open and with 21-year-old Spaniard Carlos Alcaraz claiming the French Open and Wimbledon titles, the old guard were shut out of the Slams for the first time since 2002.

"Nice to see new champions. Nice to see new rivalries," said Sinner, who tuned out a doping furore before the tournament.

"I feel it's good for the sport to have some new champions."

Sinner and Alcaraz are the only two men born after 2000 to reach a major final.

There were early clues that it would not be business as usual for the favourites at Flushing Meadows.

Strained from his victorious Olympic campaign in Paris, four-time U.S. Open champion Djokovic went down in four sets to Alexei Popyrin.

"I spent a lot of energy winning the gold, and I did arrive to New York just not feeling fresh mentally and physically," said 37-year-old Djokovic.

"But because it's U.S. Open, you know, I gave it a shot and I tried my best."

Pre-tournament concerns about how the Olympians would fare with the tight turnaround from Paris to New York and the quick switch from clay to hard courts were validated as none of the singles medallists made it past the quarter-finals.

Alcaraz, who lost to Djokovic in the energy-sapping Olympic final, also went out early, stunned by Dutchman Botic van de Zandschulp in the second round.

Olympic women's champion Zheng Qinwen, who lost in the quarter-finals, said the quick turnaround did her no favours.

"I have been always in rush since the Olympic Games," said China's Zheng. "I only had three days for prepare U.S. Open. I'm not able to do all my normal fitness program."

Hailed as the 'City That Never Sleeps', a sweltering New York put players through the wringer, the tournament featuring its latest-ever start for a match, longest-ever match and latest-ever finish for a women's match.

World number one Iga Swiatek was another leading contender who went out too soon, the five-time Grand Slam winner and 2022 champion losing in the quarters, while defending champion Coco Gauff exited in the fourth round.

With players struggling for fitness in tough conditions, it was perhaps little surprise that Aryna Sabalenka triumphed on the women's side.

The Belarusian retained her title at the Australian Open but took a mid-season break to address health and fitness concerns after suffering a back injury in Rome. She also sat out Wimbledon due to a shoulder injury.

The world number two finished runner-up a year ago and lost in the semi-finals twice before that, but there would be no New York heartbreak this time around.

"(I) had a lot of tough losses in the past," she said. "I was always hoping that one day I'll be able to hold this beautiful trophy. It's been always my dream."