After his U.S. Open quarterfinal loss to Carlos Alcaraz, Alexander Zverev praised the young Spaniard, noting that Alcaraz has risen to the level of 23-time Grand Slam champion Novak Djokovic, setting himself apart from the competition.

Zverev was once viewed as the best bet to break the "Big Three's" stranglehold on men's tennis, but the 26-year-old German has failed to kick on and is still seeking a first Grand Slam title.

With Roger Federer in retirement and Rafa Nadal likely to end his career after next season, Alcaraz has joined Djokovic at the top of the men's game, and Zverev is perhaps best placed to pass judgment on the power shift, having lost to both recently.

"I played Novak in Cincinnati (last month), I played Carlos here. They're very similar from the level of the game," Zverev told reporters after defending champion Alcaraz outclassed him 6-3, 6-2, 6-4.

"There are some things Novak does better; there are some things Carlos does better. I think they're at a level of their own at the moment."

At 36, Djokovic is playing some of his best tennis and is chasing a 24th Grand Slam title to match Margaret Court's all-time singles record.

The duo have dominated the tour this season, with the Serb winning the Australian and French Opens while Alcaraz, 20, upstaged him at Wimbledon for his second Grand Slam crown.

"The other guys have got to catch up," Zverev said. "It's as simple as that."

Zverev, who reached the semifinals at Roland Garros this year, was plagued by an ankle injury last season but said a hamstring issue put paid to his hopes at Flushing Meadows.

"I couldn't push off on my serve anymore. My serve speed was down a lot compared to other days," he added.

"Against him, I needed a good serving day; otherwise, it would have been difficult. My biggest weapon was taken away after the first set.

"It's difficult to even compete if you don't have that."