Rafael Nadal has withdrawn from the Indian Wells Masters due to a knee injury ahead of his scheduled semi-final match with Roger Federer.
The world number two was set to face Federer for a place in the final, having reached the last four without dropping a set in wins over Jared Donaldson, Diego Schwartzman, Filip Krajinovic and Karen Khachanov this week.
Nadal's knee appeared to flare up in his quarter-final win over Khachanov on Friday, where he twice received treatment from his trainer, with the Spaniard stressing after the match he would need to assess the injury ahead of the match on Saturday.
Although the second seed hoped to be fit to play, he deemed he was not in a suitable condition after a warm-up.
"I warmed up today and I felt that my knee was not good to compete at the level I needed to compete in semi-finals," said Nadal.
"It's tough for me to accept all these things that I'm going through in my career. Sometimes I feel sad because I'm in a disadvantage to all my opponents.
"[I need to] be positive and grateful for all the things tennis has given to me. I feel fortunate for all the things I've done in my life and the world of tennis."
Federer will therefore advance to the final with a walkover and face Dominic Thiem or Milos Raonic.
Nadal had been seeking his fourth victory at Indian Wells, where he last triumphed in 2013.
The 32-year-old was playing only his second event since reaching the final of the Australian Open; he lost to Nick Kyrgios in the last 16 of the Mexican Open.
Federer and Nadal were set for their 39th meeting, the Spaniard having won 23 times to his great rival's 15, although the Swiss has triumphed in their past five encounters.
After his win over Khachanov, Nadal told reporters: "I love to play on hard [courts], but probably my body doesn't love it that much."