Australia's Nick Kyrgios "lacks respect for the public, the rival and himself", says world number two Rafael Nadal.
The 17-time Grand Slam champion lost a dramatic second-round match to world number 72 Kyrgios 3-6 7-6 (7-2) 7-6 (8-6) at the Mexican Open in Acapulco.
Kyrgios served underarm late on during the match against the Spaniard.
Nadal said: "He's a player who has the talent to win Grand Slams - a player who can fight for the top rankings, but there's a reason he is where he is."
Kyrgios saved three match points before beating Nadal, who was playing his first tournament since losing to Novak Djokovic in January's Australian Open final.
"He is a very dangerous player but lacks consistency," Nadal added.
"I don't think he's a bad guy. I think he's a good guy."
Kyrgios, 23, saved nine break points in the match and came from 6-3 down in the third-set tie-break to win in three hours and three minutes, before falling to the floor in celebration.
"I'm different, Rafa's different," said Kyrgios, who will play Switzerland's Stan Wawrinka in the quarter-finals.
"He doesn't know the journey I have been through. He doesn't know anything about me so I'm not going to listen at all.
"That's the way I play. He's very slow in between points. The rule in the book says you have to play at the speed of the server. Rafa has the speed every time.
"I'm not going to comment on his game. He has got his game and I have got mine."
At the end of the first set, Kyrgios complained to a trainer about feeling ill but said he did not want to retire because he was wary of a possible backlash.
He saved four break points in the ninth game of the second set, and a further five in the sixth game of the final set.
Nadal took only one of his 10 break point chances in the match.
In the women's draw, American top seed Sloane Stephens suffered a shock 6-3 6-3 defeat by Brazilian qualifier Beatriz Haddad Maia.