Zverev was once viewed as the best bet to break the "Big Three's" stranglehold on men's tennis but the 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 judgement 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."
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 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 semi-finals 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."
