Murray will play Robin Soderling in the last eight
Andy Murray eased into the quarter-finals of the BNP Paribas Open in Indian Wells after opponent Nicolas Almagro retired with an ankle injury.
The Scot, seeded four, was leading 6-2 1-0 when the unseeded Spaniard was forced to call it a day.
Murray will play sixth-seed Robin Soderling, who beat Jo-Wilfried Tsonga in straight sets, in the last eight.
Novak Djokovic crashed out 7-5 6-3 to Croat Ivan Ljubicic, but Rafael Nadal overcame John Isner 7-5 3-6 6-3.
Ljubicic, the 20th seed, fired down 13 aces to record only his second win over Djokovic, the second seed, in seven meetings.
"I had a lot of opportunities and I didn't use many of them," said the Serb after converting only two of five break point chances.
His first serve is very, very good. He normally serves a high percentage, so it might be beneficial to play it in the wind against him during the day
"My execution was very bad and he played well when he needed to.
"I am not happy with the overall performance that I had in this tournament, because I know I could have played better and could have done better.
"But under decent circumstances I still made it to the fourth round, which is quite OK."
Almagro made an encouraging start against the British number one, breaking serve early on only for Murray to break back to love in the very next game.
And Murray imposed his authority as the world number 40 won just 33% of points on his own serve, accompanied by 16 unforced errors.
The 24-year-old called for treatment to his left ankle at the end of the first set and eventually withdrew from the match when Murray held his serve in the first game of the second set.
"He seemed like he was moving OK," Murray said.
"At the start we had some pretty good points. But then it seemed like it came on pretty suddenly.
"He said he had done it yesterday, and if you have a bad ankle sprain, it's obviously very difficult to move."
Looking ahead to his meeting with Soderling, the Scot added: "He's a very, very tough player because he's got big game serves.
"His first serve is very, very good. He normally serves a high percentage, so it might be beneficial to play it in the wind against him during the day. But he's definitely a tough player."
Third seed Nadal had to deal with a partisan home crowd and his opponent's serve-and-volley game in a match played in searing afternoon heat.
Eventually the Spaniard earned his place in the next round with a forehand winner into the open court, which he greeted with a loud yell of relief.
The two-time winner, who beat Murray in last year's final, will face the Czech Republic's Tomas Berdych, who thrashed Serbia's Viktor Troicki 6-1 6-3.
"I am feeling really well and I am feeling good on court," said Nadal.