John McEnroe insisted Nick Kyrgios has an "opportunity to be something spectacular", with the former world number one believing the Australian star can become the best player on the ATP Tour.
Talent has never been an issue for 21-year-old Kyrgios, who opened his Wimbledon campaign with a 6-4 6-3 6-7 (9-11) 6-1 victory over veteran Radek Stepanek on Tuesday.
Kyrgios' fiery temperament, however, has caused the world number 18 problems in the past, following a number of on-court incidents.
But three-time Wimbledon champion McEnroe - renowned for his outspoken and tenacious style - said Kyrgios has all the attributes to reach the summit of men's tennis.
"He's got an opportunity to be something spectacular," the 57-year-old American told BBC 5 Live Sport.
"I really believe he can be the best player in the world.
"If he gets his head together and figures out how... I mean I saw him today where it looked like late in the third set after he'd blown a few match points, to me it looked like he stopped trying. When he was still like a couple of points from winning the match."
Next up for 15th seed Kyrgios - yet to make it past the quarter-finals of a grand slam - at SW19 is Dustin Brown in a battle of the entertainers.
McEnroe added: "He eventually pulled it together and won in the fourth set and he was playing Stepanek who knows a thing or two about getting under your skin and knows how to play on grass, but if he is that fragile he's not going to win majors.
"We'll have to wait and see on that one. The jury is still out."