Del Potro, the 2012 runner-up and 2013 champion, beat Paul-Henri Mathieu of France 6-2, 7-6 (2) but had to save two set points on serve at 5-6 in the second.
"I recovered myself well in the tiebreak, because I made too many mistakes in the second set," he said. Still feeling pain in his left wrist, he added, "Sometimes I get frustrated ... it's not easy when I want to close the point and hit hard but I have to hit slices. But I have done this for the past month on the tour and I have to deal with it. If the pain goes away I will start to hit the backhand harder again."
Second-seeded Murray bested Austrian qualifier Dominic Thiem 6-4, 3-6, 6-3. They were evenly matched in the first two sets, with Thiem saving three break points to take the second. But Murray asserted himself in the third, in an exciting match which lasted 2 hours, 21 minutes.
"It was unbelievably tough with a lot of long games," Murray said. "Maybe fun for the fans, but not (for me) to play against a talent like Dominic Thiem. We'll be hearing a lot from him."
Also, third-seeded Tomas Berdych beat Nicolas Mahut of France 6-4, 6-3 and will meet Wimbledon semifinalist Jerzy Janowicz on Friday. But fourth-seeded Richard Gasquet was knocked out by Philipp Kohlschreiber 7-5, 7-5, and the German will face Dutch wild card Igor Sijsling in the last eight.