Rafael Nadal is unsure how he will wake up in the morning as the world number two cast doubt over his Indian Wells Masters blockbuster against Roger Federer due to a knee injury.

Nadal set up a 39th career meeting with old foe Federer following his 7-6 (7-2) 7-6 (7-2) quarter-final victory over Karen Khachanov on Friday.

The trainer attended to Nadal twice during the match as the 17-time grand slam champion's knee appeared to flare up in the California desert.

Asked if he would be okay for Saturday's showdown, Spanish star Nadal replied: "I hope.

"It is difficult for me to answer, because anything I say can be taken negatively, nobody can guarantee anything that will happen the next day because, in this world, anything is possible.

"But of course my goal and my idea is to be ready for tomorrow, I cannot guarantee how I'm going to wake up tomorrow."

Australian Open runner-up Nadal added: "I love to play on hard [courts], but probably my body doesn't love it that much.

"As I say before, is not only about today, is about tomorrow. Maybe we're going to pay the price at the age of 45 or something. That will be not nice. When I see some old legends walking around the tour, is tough to see.''

Nadal leads the head-to-head record against Federer 23-15, though the Swiss great has won the past five meetings – including his victory in the 2017 Shanghai Masters final.

"When Rafa is healthy, especially in a place like here in Indian Wells where he feels comfortable, he's clearly very tough to play," said Federer, who beat Hubert Hurkacz 6-4 6-4.