John McEnroe once lashed Nick Kyrgios for being a "bonehead" and a "clown". Now tennis's original super-brat is hailing the new-age version as a coachless "genius" as Kyrgios closes in on grand slam glory in New York.
Kyrgios enters his US Open quarter-final against Russian Karen Khachanov on Tuesday (11am Wednesday AEST) as the new men's title favourite following an electric fourth-round win over world No.1 Daniil Medvedev and the subsequent demise of second seed Rafael Nadal.
And McEnroe can barely believe the transformation in the Australian, who, at 27, is finally pulling it all together after years of being the sport's most prodigious - and unfulfilled talent.
"I am on a high, to see him be this professional," McEnroe told Eurosport at Flushing Meadows.
"He's 27 and potentially there are a bunch of years where he can do his thing, and boy did he do it last night.
"The guy is a genius on the court. He doesn't need a coach, he did an unbelievable job tactically.
"He played a lot of matches where he wasn't giving it his all and now you see what he is capable of when he does, when he trains, when he is in shape. The guy is phenomenal."
His eyes firmly on the prize, a home-sick Kyrgios teased said after taking down Medvedev about having "three more matches potentially, then we never have to play tennis again".
McEnroe said "it's not impossible to think that he could do that".
"We hope he doesn't do that because the phrase 'better late than never', that applies to Nick because he wasn't professional for a long time."
The former world No.1 and four-time US Open champion was rarely more critical of Kyrgios than during a first-round loss to Andy Murray in New York in 2015.
Seemingly more interested in entertaining the crowd, the flashy talent infuriated McEnroe for attempting a tweener instead of hitting a routine volley winner.
"I hope he learns from these bonehead moves. He should be a seed already, at least 20. You don't want to be remembered as a clown. You want to be remembered as a player.
"So he better step up. Otherwise he won't be talked about too much. He thinks he's a vaudeville entertainer. I don't know what's going on here."
Fast forward seven years and Kyrgios is the talk of the tournament, favourite to emerge from a quarter-final line-up devoid of grand slam champions to claim an elusive maiden major.
"If he plays like he did on Sunday night, he is going to win the tournament." McEnroe said.
If he beats Khachanov, at the very least Kyrgios will have earned a semi-final shot on Thursday (Friday AEST) against either world No.7 Casper Ruud after the Norwegian outclassed Italian 13th seed Matteo Berrettini 6-1 6-4 7-6 (7-4).
