''You got the back against the wall and hope to get a bit lucky. And you hope to play exactly the right shots that you need,'' Federer explained. ''Or that he completely just messes it up. Either way works, as long as you get out of it.''
Federer got out of it. Twice.
Steady as ever, even at 33, Federer saved two match points en route to coming all the way back from a two-set deficit, edging the 20th-seeded Monfils 4-6, 3-6, 6-4, 7-5, 6-2 on Thursday night to reach the semifinals at Flushing Meadows for the first time since 2011.
Frustrated by the Frenchman's unpredictable style, flummoxed by the swirling wind, and missing shots he normally makes, Federer was on the verge of defeat while trailing 5-4 in the fourth set.
The 17-time major champion's mind was racing.
''That is a very frustrating moment to be in. Being down match point, it's just not fun, because you're so close to leaving the court, head hanging down and ... going to take a shower and going to have to do press and all that stuff, which is so annoying after you've lost,'' Federer said.
''It's hard to block it out, (but) you snap right back in, because you don't have that much time,'' he continued. ''You're like, 'OK, let me try and hit a good serve. Let's hope it works, because I don't want to hit a second serve.' All that kind of stuff. ... I have to face it and embrace it.''
That he did.
With Federer serving at 15-40, Monfils had an opening for a backhand passing shot, but it flew long. At 30-40, Federer produced a forehand winner, and the crowd roared. Two points later, it was 5-all, and then Monfils double-faulted twice in a row to get broken.
Everything had changed, putting Federer on course to winning for the ninth time after dropping the opening two sets of a match.
''It came quick,'' said Monfils, who said he felt physically and mentally drained late.
''It's a matter of five minutes,'' he said. ''I think I was down (for) five minutes. Roger just (jumped) on me.''
Afterward, Federer credited the raucous support he heard in Arthur Ashe Stadium, saying the spectators ''definitely got me through the match.''
''It grows your belief that you can hit better shots, you can dig out more tough balls, you can serve better. All that just helps solidify your belief,'' Federer said. ''I must say tonight was actually quite emotional for me.''
Monfils, 28, was trying to reach his second career Grand Slam semifinal. Instead, Federer advanced to his 36th, ninth at Flushing Meadows. Five of Federer's major titles came at the U.S. Open from 2004-08, but he exited in the quarterfinals in 2012, and the fourth round in 2013.
On Saturday, the second-seeded Federer will play 14th-seeded Marin Cilic of Croatia. The other semifinal will be No. 1 Novak Djokovic against No. 10 Kei Nishikori.
Cilic wasn't allowed to play in last year's U.S. Open, forced to the sideline by a four-month doping suspension that he says he didn't deserve. But Cilic reached the second Grand Slam semifinal of his career, and first since 2010, by beating sixth-seeded Tomas Berdych of the Czech Republic 6-2, 6-4, 7-6 (4) Thursday.
It took Monfils 78 minutes to build a two-set lead, shaking of a twisted right ankle that left him down on the court. It helped that Federer made 26 unforced errors in that span.
And as if all he had to do was want to improve his play, Federer did. He suddenly was remarkably cleaner, with only one unforced error in the third set, and two in the fifth.
Attacking the net helped: Over the final three sets, Federer earned the point on 34 of 46 trips forward.
''I knew I could play better after the first couple of sets,'' Federer said. ''I believed I could turn it around from the get-go when the third set started.''
Monfils is nothing if not unpredictable. In an era where some men, including Federer, have two coaches, Monfils goes without any. He'll admit to tanking points, games or entire sets. He sips sodas during matches, including Thursday. He'll go for a between-the-legs shot when a mundane forehand would do.
Make no mistake, though. This was not easy. The turnaround might not have been possible for Federer a year ago, when he was dealing with a bad back and trying to figure out whether he should switch to a larger racket head.
But now Federer, however old, is approaching his skills of old. He got to the finals at his previous four tournaments - including a loss to Djokovic in Wimbledon's title match in July - the first such run by a 30-something since Ivan Lendl in 1990. One more victory, and Federer's final streak will stretch to five in a row.
Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich
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- Roger Federer
- Gael Monfils