Eighth seed Jurgen Melzer was forced to retire from the Malaysian Open Monday after picking up a thigh injury in the first round, meaning Vasek Pospisil, ranked 122 in the world, went through.
Melzer, 37 in the world rankings, was trailing 6-4, 2-0 when he called it a day at the ATP 250 tournament and did not speak to reporters after the match at the Putra Stadium on the outskirts of Malaysia's capital Kuala Lumpur.
Canadian Pospisil said he was "disappointed" his Austrian opponent was unable to finish the match but felt "confident" after the victory.
"I will enjoy the win but it is not how I wanted to win... If he had carried on and finished the game, I would have considered it the best win of my career and a good achievement," Pospisil, who is looking to break into the top 100 by the end of the year, said.
It was unclear whether Melzer would be able to compete in a doubles match scheduled for Tuesday.
Monday's first round also sprung an upset for Brian Baker, who lost 6-3, 7-6 (7/4) to fellow American Michael Yani.
Baker, the world number 60, made a remarkable comeback in 2011 after six years out with multiple injuries and surgeries.
Yani, who ranks 238, will now meet Pospisil in the second round of the $947,750 tournament.
Malaysia's challenge came to an end when Ariez Elyaas Deen Heshaam was knocked out by Dutchman Igor Sijsling 6-2, 6-1.
Taiwanese Jimmy Wang booked a date with world number 11 and second seeded Juan Monaco after beating Italian Riccardo Ghedin 6-3, 6-3.
"I have not played Monaco before but I have watched him play a few times... It will be tough, but I think I can probably cause an upset," Wang said.
Spain's world number five David Ferrer is the top seed in the competition, which ends on Sunday.