Nick Kyrgios calls Roger Federer the GOAT.
The enigmatic Aussie also insists Federer is insane.
Watch: Federer's New Uniqlo Deal
Asked his thoughts on the epic 2008 Wimbledon final between Rafael Nadal and Federer, Kyrgios said the level both champions are producing in their mid 30s is "insane."
"Yeah, it's insane," Kyrgios told the media after his four-set win over Denis Istomin in his Wimbledon opener today. "Like tennis is physical. I get sore after I played like three matches in a row. I don't even know if I can play, like, when I'm 28. I don't know how old he is, like 36. That's insane. It's crazy. I mean, they're so professional. They do everything, tick all the boxes. Too good."
Federer bunted a backhand pass on match point out-dueling Kyrgios, 6-7 (2), 6-2, 7-6 (5), in last month's Stuttgart semifinals.
The 15th-seeded Aussie warmed up for Wimbledon with back-to-back semifinal runs in Stuttgart and Queen's Club.
Today, Kyrgios cracked 42 aces in a 7-6 (3), 7-6 (4), 6-7 (5), 6-3 victory sending him into a second-round meeting with Dutchman Robin Haase.
The explosive Aussie admits he's engaged in a constant mental tug-of-war during major matches.
"I just have so many thoughts when I'm out there," Kyrgios said. "Like, I get so angry. I'm, like, I just go through so many different patches in a game. I guess it's so hard for me to find that balance...
"I look like I don't care one minute, then the next minute I'm playing really well. Not much really goes on, to be honest. It's just like, I don't know. It's a tug of war all the time."