'It feels good, ' said the 37-year-old afterwards. It is maybe not something I'm that used to lose and come back and play again, but it was a good exercise.
First up on court, it was Kei Nishikori, who took on Kevin Anderson for a third tournament in a row and it was the big man from South Africa, who took some sweet revenge of his defeat in Paris, with a scintillating 6-0, 6-1 win, to seal his second consecutive victory at the Nitto ATP Finals. "He deserved that win at Wimbledon but hopefully I'll get my revenge", Federer said of his Anderson rematch. "So, from my side, I did a fantastic job throughout really".
"My first-round match here wasn't great, so today was better, and I need to keep that up against Kevin if I want to have a chance to stay here in London", Federer added.
Anderson, the Wimbledon finalist, swiftly completed his 47th win of the season, topping his career-high from 2015.
"I just felt I returned really well, made him hit a lot of shots off his serve".
"I was growing in confidence and he sort of probably was going a little the other way, which is tough given the situation".
After suggesting that the whole debate has been taken out of context, Federer told reporters there is no obvious pattern in the way the timing of his matches is arranged.
Federer recovered from a woeful performance in his opening match against Kei Nishikori on Sunday night by maintaining his record of never losing his second round-robin match in this tournament, although he did not have to work too hard for it.
Thursday will see members of Group Lleyton Hewitt fight for their lives to advance to the semi-finals here at the O2 in London, with all four players still in with a shout of qualifying. "Maybe it's the round-robin format that got to me", Federer said.
Thiem, who lost his opener to Anderson, saved a break point in the first game but Federer kept him under constant pressure and broke at 1-1 when the 25-year-old sent a forehand long.
The ATP Finals is contested by the eight players who have accumulated the most ranking points over the season separated into two groups, with the best four players reaching the knockout semi-finals stage.