A candid Nick Kyrgios said rising to the big occasion was never likely to be a problem after his quarter-final victory over Rafael Nadal at the ATP Western and Southern Open, but conceded he has been guilty of tanking against lesser opponents.

In his second match of the day in Cincinnati on Saturday (AEST), Kyrgios turned in a hugely impressive display to triumph 6-2 7-5 against Nadal, who will return to the summit of the world rankings on Monday.

After his victory, the mercurial Australian was asked to comment on his apparent ability to save his best performances for matches against the likes of Nadal.

"Obviously I think it's easier to say that for me because you see me tank ridiculous amounts of matches ... on back courts and stuff like that, and you never see me doing anything silly obviously in a match like this," Kyrgios said. "It's easy to get up for these matches.

"A little kid playing on the centre court of Cincinnati against Nadal. That's the way the best has to come out.

"The problem for me is trying to bring it on an everyday basis, say the centre court of Lyon with, like, 15 people against Nicolas Kicker earlier this year. I lost. Tonight I'm playing Rafa and I win.

"These matches aren't the problem. It's those [low-profile] matches for me."

Kyrgios also opened up about the off-court problems he has faced in recent times, including the death of his grandfather.

"In the last couple of months I had a rollercoaster of scenes," he said.

"Ever since Davis Cup [when he led Australia to the semi-finals in April], [I] had such a big high. Obviously my grandpa passed away. Had some off-court issues. My head's been all over the place.

"I have just been going through a load of s***, sorry for cussing, but a load of stuff that's distracted me from just trying to play some tennis, and hopefully I'm just trying to get on the right track again." 

Nadal acknowledged Kyrgios was a worthy winner on Saturday (AEST).

"He, obviously, is a tough match always," the Spaniard said. "Nick is a great player, but I played poorly.

"I started the match well and then I played a very bad game in the second set when he broke me. I played some good points. He had some mistakes and I was able to be back in the match. And then I played a terrible game. So it was a bad match for me. Congrats to him. That's it."