Rafael Nadal was forced to dig deep to see off a player of 'unbelievable potential' as the world No.3 booked his place in the quarter-finals of the BNP Paribas Open in Indian Wells.
Nadal, who is returning to action at this tournament following several weeks on the sidelines with a knee injury, required three sets to see off the challenge of giant American John Isner.
The defending champion was looking to avoid joining world No.1 Roger Federer through the exit door, and survived a scare against 6ft 9in Isner to earn a 7-5 3-6 6-3 win in two hours 10 minutes.
"Today was a really difficult match, so I know I have to play my best tennis," said Nadal.
"I did really well, I think. I only played a really bad game in the fourth game of the second set, but the rest of the match I'm very happy how I did."
"I think John is a big player, very difficult to play against him, and he has unbelievable potential. So for me it is a very important and very good victory."
Nadal will now meet the Czech Republic's Tomas Berdych, who defeated Viktor Troicki of Serbia 6-1 6-3.
Nadal's hopes of retaining his title were boosted on Tuesday after Federer was ousted by Marcos Baghdatis, but the Cypriot failed to build on that victory as he crashed out at the hands of 18th-seeded Spaniard Tommy Robredo, 7-5 0-6 6-4.
Second seed Novak Djokovic was the major shock of the round as he crashed out following a 7-5 6-3 loss at the hands of Ivan Ljubicic, the 20th seed.
The Croatian secured three breaks of serve to oust the Serbian and progress to the quarter-finals.
Despite the disappointment of defeat, Djokovic maintained he could take some positives from his efforts this week.
"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," he said.
"But under decent circumstances I still made it to the fourth round, which is quite okay."
Andy Murray eased into the quarter-finals after fourth-round opponent Nicolas Almagro was forced to retire with an ankle injury.
Fourth seed Murray was leading 6-2 1-0 when his unseeded Spanish opponent was forced to call it a day having already called a medical time out at the end of the first set.
Next up for Murray is a tricky looking encounter with sixth-seeded Swede Robin Soderling, who defeated Jo-Wilfried Tsonga 6-3 6-4 to reach the last eight.
Seventh-seeded American Andy Roddick is also through courtesy of a 7-6 (7-5) 6-4 outcome against Austria's Jurgen Melzer, while Argentina's Juan Monaco set up an encounter with Djokovic's conqueror Ljubicic after securing a 3-6 6-2 6-1 over Spain's Guillermo Garcia-Lopez.