LONDON, ENGLAND - JUNE 19: Nick Kygrios of Australia plays a backhand during the mens singles first round match against Donald Young of The United States during day one of the 2017 Aegon Championships at Queens Club on June 19, 2017 in London, England. (Photo by Julian Finney/Getty Images)

American tennis great, John McEnroe, believes Australian youngster Nick Kyrgios is the most talented young player on tour.

McEnroe says of all the youngsters competing to make the leap and push the big four, Alexander Zverev and Nick Kyrgios are the two who will have the biggest impact on tour, with Kyrgios possessing the tools to win several Grand Slams.

“Zverev is the guy in a year or two. I think he will be, but I don’t think he’s ready to win it yet,” McEnroe said.

“Kyrgios would be the most talented guy."

However, McEnroe questioned Kyrgios' mental application during matches, suggested it could hold him back.

"I don’t know what’s going on with him. I know I get frustrated with his lack of effort at times," he said.

“The talent is enough to win multiple majors. The raw tennis talent. But you have to combine everything.

“Kyrgios, grass would be his best surface, his best chance to win one.

“There’s just so many question marks with him, not only mentally, but physically as well.”

Kyrgios is the 20th seed at Wimbledon this year, while Alexander Zverev is the 10th seed.

However, McEnroe says Milos Raonic and Grigor Dimitrov are two who could challenge the top four at Wimbledon this year.