NEW YORK (Reuters) - Austrian second seed Dominic Thiem advanced to the second round of the U.S. Open on Tuesday when Spaniard Jaume Munar retired after dropping the first two sets.

Thiem, who reached his third Grand Slam final at this year's Australian Open where he lost to Novak Djokovic, was leading 7-6(6) 6-3 when Munar, who had slipped on the baseline midway through the second set, ended the match.

"I was happy with the way I was fighting, because it wasn't easy to play him," Thiem said during an on-court interview inside Louis Armstrong Stadium.

"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 came into the match having won just three games in his shock first-round exit at the U.S. Open tune-up event, which also marked his first match since the ATP Tour returned from its COVID-19 hiatus.

Thiem, who had dropped just one set in three previous meetings with Munar, broke the Spaniard three times and hit 33 winners in the one-hour and 55-minute encounter.

Up next for Thiem will be a showdown with India's Sumit Nagal, who beat Bradley Klahn 6-1 6-3 3-6 6-1.

Thiem has never faced Nagal before but remembers clearly what his next opponent is capable of having watched him in his Grand Slam debut last year in New York where he took the opening set against Roger Federer before going on to lose the match.

"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."

(Reporting by Frank Pingue in Toronto; Editing by Toby Davis)