John McEnroe has urged Nick Kyrgios to focus fully on his game if he wants to become a future Grand Slam champion.
The volatile 20-year-old Australian faces a tough task to make progress at the upcoming US Open, having been drawn to play former champion Andy Murray in the first round.
And McEnroe is concerned that the former Australian Open and Wimbledon quarter-finalist is allowing outside factors to influence his play, when he needs to be fully focused on the task in hand.
He told the New York Times: "He could be incredibly positive for our sport.
"He's got a chance to be top five, win Grand Slams, but he's rubbed people the wrong way and he's going to get railroaded out of town if he doesn't watch out.
"I worry that this is going to take away from what he needs to be doing, which is learning from losses, training hard, learning how to gauge matches mentally, all the things you need to learn to become a Grand Slam champion.
"I'm telling you, there's something about this guy. He's a natural, real charisma."
Kyrgios has already faced Murray in two of three Grand Slams played earlier in the year, losing in straight sets on both occasions - in the last eight in Melbourne and the last-32 at Wimbledon.
Their only other meeting came at the Rogers Cup in Canada last year, when Murray also prevailed in straight sets.