Dominic Thiem advanced was leading 7-6(6) 6-3 when Jaume Munar retired.
"I was happy with the way I was fighting, because it wasn't easy to play him," Thiem said.
"The courts are very different this year, the court seemed to be slow inside. It was a little bit windy, but we had some good rallies.
"I felt I was playing good. I played a good tiebreak, which was pretty comfortable, then I'm sorry for what has happened."
Munar came back from 5-3 down in the opener to pull ahead but Thiem held to love to force a tiebreak which he went on to secure on his second set point when he fired down an ace.
After Munar held serve to level the second set at 3-3, Thiem stepped up his game and won 12 of the next 16 points to seize control before the Spaniard retired after one hour, 55 minutes.
Thiem broke the Spaniard three times and hit 33 winners in the one-hour and 55-minute encounter.
He will next play Sumit Nagal, who beat Bradley Klahn 6-1 6-3 3-6 6-1.
"I actually saw the full match. He was playing well," said Thiem. "I just remember that he has an amazing forehand, like a really, really good one.
"So maybe I'll see some highlights of that match, some highlights of his match today."
Photo by Pete Staples/USTA
