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NEW YORK (AP) Ernests Gulbis was surprised by how conflicted he felt during his second-round loss at the U.S. Open.

That's because his foe, Dominic Thiem, also happens to be a good pal and frequent practice partner who works with the same coach. They even warmed each other up earlier Friday, before heading out on court for the real thing.

After taking the first two sets, No. 11 Gulbis blew all of that big lead and became the first man seeded in the top 20 at Flushing Meadows this year to exit, bowing out against the 45th-ranked Thiem 4-6, 3-6, 6-4, 6-3, 6-3.

"I thought that it's not going to be so difficult. But mentally, it was difficult," said Gulbis, who beat Roger Federer on the way to reaching his first Grand Slam semifinal at the French Open.

"We were both really nervous throughout the match, I think. We both couldn't play ... our best tennis," Gulbis added. "I mean, the level we play in practice is much higher than we did today."

Gulbis said that late in the third set, he thought he might have injured his hamstring, but it turned out to be cramps that were affecting his ability to move.

Asked whether he continued playing because he did not want to hand over the victory to Thiem, Gulbis answered that his thinking was the opposite: If he was going to stop against any opponent, it would have been his close friend.

"But I don't like to retire if I can finish the match," said Gulbis, who made nearly twice as many unforced errors as Thiem, 78-41.

This was the first match of Thiem's career that went five sets.

And it allowed the 20-year-old the youngest player ranked in the ATP's top 50 to reach the third round of a Grand Slam tournament for the first time.

"I would be happier if it would have happened against someone else," said Thiem, who will face 19th-seeded Feliciano Lopez of Spain next.

"I hated the situation," Thiem said.

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Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich