Story highlights Rafter says Kyrgios needs mental preparation

World No. 13 Kyrgios can win Wimbledon says Rafter

Kyrgios still competing without a coach

London (CNN) Australian tennis bad boy Nick Kyrgios has what it takes to be world No. 1, according to a man who reached the top of the sport.

Pat Rafter, a two-time Wimbledon finalist, also backed his compatriot to one day win the grass court grand slam.

"I know that talent-wise, he's as good as I have seen in the last 20 years," the twice US Open champion told CNN's Open Court. "He can be No. 1 in my mind."

Rafter -- who retired in 2002 before a brief comeback in 2014 -- admits, however, that the unpredictable Kyrgios needs to combine a disciplined lifestyle with mental toughness if he is to make good on his potential.

"He's a real mystery to me at the moment," said Rafter, who last week took part in the Champion's Tennis tournament at London's Royal Albert Hall. "I don't know how he's coping with things mentally."

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