NEW YORK -- When Roger Federer smacked a backhand return winner off his opponent's 142 mph serve Friday night, he glanced at the speed readout, just to find out exactly how difficult what he'd done had been.
Then, later in his 6-4, 6-4, 6-4 victory over big-hitting Sam Groth in the US Open's second round, Federer again checked the digital display in Arthur Ashe Stadium after getting a 147 mph offering over the net en route to taking that point, too.
The 17-time major champion said that while he couldn't know for sure what the precise speeds were, he's usually pretty good at guessing.
"I'm aware of every serve, how hard it is after the point when he goes big, because I think it's interesting and I want to see," Federer said. "Sometimes I feel it, as well. I'm like, 'That felt like 138,' and it might be just a couple off. It's the same with my own serve. I can judge it probably [within] a few miles an hour."
A reporter jokingly asked Federer whether his eyes were open when he got his backhand on the fastest serve of the match.
"Got to check the replay. I'm not sure. It was maybe one of those moments," Federer said with a laugh as he shut his eyes and pantomimed stretching out his right arm in hopes of making contact with the ball.
Easy to have fun after an easy victory, right?
In the third round, Federer -- who has won five titles at the US Open -- will face 42nd-ranked Marcel Granollers of Spain.
The second-seeded Federer improved to 54-1 in second-round Grand Slam matches; the only loss came at Wimbledon last year. He also is now 24-1 over his career in matches under the lights in Ashe.
"I was trying not to get caught up in the whole 'Roger act' out there. You walk out, you get a few cheers. He walks out, and the crowd goes ballistic," Australia's Groth said. "So from the word 'Go,' you know he's there."
Doing an impressive job of handling the 104th-ranked Groth's powerful serves, Federer limited him to eight aces and broke him five times.
During his on-court interview, Federer was asked about dealing with such fast-moving serves and he said, kiddingly, "My arm's still vibrating a little bit."
Federer also produced a highlight-reel shot in the second set's final game, closing a 17-stroke exchange by sliding to his backhand side, stretching and flicking a high-arching lob that landed in. Federer raised his right hand overhead and wagged his index finger in a "No. 1" gesture.
"I knew I was playing Roger Federer," Groth said. "I was pretty nervous probably all day."
Eleventh-seeded Ernests Gulbis became the first man seeded in the top 20 at this year's US Open to lose, blowing a two-set lead in the second round against his pal and frequent practice partner, 45th-ranked Dominic Thiem of Austria.
Thiem won 4-6, 3-6, 6-4, 6-3, 6-3 in a match between players who share a coach, Gunter Bresnik. They even warmed each other up earlier Friday, before heading out on court for the real thing.
"I thought that it's not going to be so difficult. But mentally, it was difficult," said Gulbis, who beat 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. 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."
Sixth-seeded Tomas Berdych was pushed to five sets before moving on to the third round.
Berdych beat 65th-ranked Martin Klizan of Slovakia 6-3, 4-6, 6-2, 3-6, 6-3 in 3 hours, 45 minutes.
Klizan double-faulted on break point to give Berdych a 3-1 lead in the final set. Berdych then saved a break point in the next game with a big serve.
He fought off two more break points in the last game to serve out the match. Klizan had 66 unforced errors.
Tomic had surgery on both hips in January. He said the discomfort he felt starting in the first round wasn't in the same spot, but he didn't want to risk aggravating it, especially against somebody like Ferrer who runs his opponents around the court.
Also advancing were seventh-seeded Grigor Dimitrov, who beat Dudi Sela 6-1, 6-2, 6-2; 12th-seeded Richard Gasquet, who beat Paolo Lorenzi 7-6, 6-3, 6-3; and 20th-seeded Gael Monfils, who defeated Alejandro Gonzalez 7-5, 6-3, 6-2.
The Associated Press contributed to this report.