Alexander Zverev has admitted that he is not sure if Novak Djokovic is prepared to work hard enough to return to his brilliant best. The veteran Serb has failed to win a single tournament since prevailing at the Olympics last year. He came agonisingly close at the Miami Open two months ago but suffered a shock defeat to Jakub Mensik in the final.

Since then, Djokovic has slipped to early exits in each of his last two tournaments. At the Monte-Carlo Masters, he was beaten in straight sets by Alejandro Tabilo before losing in a similar fashion to Matteo Arnaldi at the Madrid Open. His poor recent form has cast doubt over whether he will ever be able to restore former glories.

Zverev, who is one of Djokovic's closest friends on tour, was quizzed on the 37-year-old's current plight after his victory over Vilius Gaubas at the Italian Open. Djokovic is not playing in Rome, having withdrawn from the tournament before the first round.

"It's only here that he pulled out, right? He played all the other tournaments," said Zverev. "Maybe he didn't play up to his standard or his liking, but who does sometimes.

"If you're not winning the tournament, if you're a top guy and you're not winning the tournament, you always go home a little bit p***** and a little bit upset. I still believe once he finds his game, he's still one of the most dangerous players in the world.

"There's no question about his ability at all. It's more whether he still wants to. It's more whether he still wants to put in the work. That's a question to him. I can't answer that."

Zverev is not the only tennis figure to have questioned Djokovic's work ethic over the last few months. Patrick Mouratoglou, who coached Serena Williams for over a decade, recently issued a stinging review of his abject displays in Monte-Carlo and Madrid.

"It's like that for a guy like him, if he doesn't have the motivation, then he shouldn't even be playing, because it seemed like he wasn't even trying to win," said Mouratoglou.

"I'm not saying he's not capable of playing better, he certainly is, but I feel like he's physically not ready. Tennis-wise, physically, for such tournaments, especially because there is no motive.

"The question arises, why did he play them then? I don't understand that. Motivation is a big thing for him. Everything revolves around that and I've been saying that for a while."