i think it has happened quite a few times that fatigue has been a factor for a defeat, perhaps not the deciding or crucial factor but certainly a factor that influenced his performance. the thing with federer is that it doesn't show -i.e. he could look totally fine but in reality he's tired and fatigued.
here's an interesting article about his training:
Training With Federer in Dubai
By GEOFF MACDONALD
Ever wonder how Roger Federer trains? I spoke with Jeremy Bayon, a Frenchman who played college tennis at Mississippi State and is now the assistant coach of the men’s team at the University of Florida, about his experience watching Federer train in Dubai with Jesse Levine, a young American player Bayon coaches. (In the second round of the Australian Open on Wednesday, Federer survived a five-set match against Gilles Simon of France, 6-2, 6-3, 4-6, 4-6, 6-3.)
Bayon and I talked during last spring’s N.C.A.A. tournament in Athens, Ga. Most coaches love to talk shop, to pick each other’s brains in an attempt to learn, and I was intensely curious to learn more about Federer’s practice routine.
“It was a training camp in the heat of July in Dubai,” Bayon said. “Very hot and only a little breeze. Aside from Jesse, Ricardis Berankis was also invited, and Yves Allegro, a Swiss player and friend of Federer, would sometimes come in the late afternoon for more set play. The practices were very focused, well-organized and simple.”
The players practiced for 12 days, with the Swiss Davis Cup captain Severin Luthi and Federer’s physical trainer, Pierre Paganini, on hand to help with the planning of each practice session.
As Bayon remembers, “Each day we would get picked up at our hotel, usually by Severin, but a few times by Federer and twice by his wife, Mirka, and driven to a hotel with tennis courts. It was extremely hot, so very few tourists were in Dubai. We would see Federer already on the court, running sprints as he finished a hard one hour conditioning session with Paganini. He would then rest for a few minutes while Jesse and Ricardis jogged and stretched. Then he would join them on court and warm up too.”
The players would begin with two-on-one drills, with Federer working alone against the two younger players. Levine, a lefty with a long, flowing topspin forehand, was invited as a stand in for Nadal. He and Berankis would play from the baseline for a while, and then, to increase the intensity and shorten the rest interval between shots, come to the net and volley to Federer.
Bayon was struck by how Federer began the drill in a playful, relaxed way, but then, almost imperceptibly, he would shift into deep concentration.
“One minute he was hitting easily,” Bayon recalled,”and then — Boom! — He became the Roger Federer you see in a Grand Slam final. He completely shuts out all distractions, and for 15 straight minutes he was in the zone. He ran for every ball, even out balls.” The trio would then take a drink break, during which Federer would sometimes ingest an energy gel. They would then resume doing two on one drills, going hard for close to an hour.
The second phase of practice was set play. Federer would play for 60 to 90 minutes, first with Levine, followed by Berankis. Allegro would sometimes come later, if Federer wanted more set play.”What impressed me the most was that he revealed nothing. I never saw fatigue. After practice, Federer would say,’I am completely exhausted’, but he looked the same. After set play he always hit two small baskets of serves –one to the deuce, and one to the ad- for a total of 100 serves.”
In the evenings, at dinner back at their hotel, Levine and Bayon would marvel at the simplicity of Federer’s approach.
“There weren’t any fancy drills or games, just lots of two-on-ones and set play. Federer played on average five sets a day, this after a conditioning session with Paganini and an hour of drilling,” Bayon said. ”His work ethic just blew us away.”