John McEnroe blasted Nick Kyrgios as a "bonehead" following the Australian's latest antics in his US Open first-round defeat to Andy Murray.
World number three Murray proved too strong for Kyrgios as the two-time grand slam champion, a winner at Flushing Meadows in 2012, claimed victory 7-5 6-3 4-6 6-1.
Kyrgios has regularly courted controversy in 2015 and was fined by the ATP last month after a Rogers Cup match with Stan Wawrinka in which he told the Swiss that fellow Australian Thanasi Kokkinakis had slept with his girlfriend.
The 20-year-old has recently turned to former world number one Lleyton Hewitt to mentor him, but an otherwise relatively impressive performance from Kyrgios was overshadowed by more on-court histrionics at Arthur Ashe Stadium.
Kyrgios appeared to try to take a nap during one of the changeovers, smashed his racket and launched an expletive-laden rant at umpire Carlos Ramos for allowing fans to enter the stadium during one of his service games.
But it was Kyrgios' failed attempt at a between-the-legs half-volley as he pressed to try to break Murray's serve in the second set that drew the ire of four-time US Open champion McEnroe.
McEnroe said of the shot on ESPN: "An absolute brain freeze. Absolute stupidity. He could have volleyed that away for another break-point chance.
"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."