NEW YORK (AP) -- When Roger Federer smacked a backhand return winner off his opponent's 142 mph (229 kph) 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 U.S. Open's second round, Federer again checked out the digital display in Arthur Ashe Stadium after getting a 147 mph (237 kph) 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 U.S. 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's also 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.''
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