Or was it the court?
The 20-time grand slam champion eased to a straight-sets win over Frenchman Adrian Mannarino on Monday to book a place in the quarter-finals at SW19.
The victory saw him become only the third man to come from two sets down to beat Federer, following in the footsteps of Jo-Wilfried Tsonga and Novak Djokovic, leaving the Swiss to ponder how he allowed the defence of his title to come to an end.
In the fourth set, Federer and Anderson alternated winning the first six games until Anderson broke Federer's serve to take a 4-3 lead and then Anderson went up 5-3 thanks to strong serving before he clinched the set.
And he didn't stop there. Anderson and Federer are knotted at 3-3 in the final set.
Federer fell to 266-3 in career Grand Slam matches where he won the first two sets.
Djokovic had told reporters that he had been hopeful of playing his quarter-final match on Centre Court before telling Serbian media that he "deserved" the honour and meant to lobby the All England Club.
"I just kept on telling myself, 'I have to keep believing.' I kept saying that today was going to be my day", Anderson said, "because you really need that mindset taking the court against somebody like Roger".
Since his last defeat at Wimbledon in 2016, Federer had racked up a run of 32 consecutive sets without loss, aiming for a Wimbledon record 35 in today's match.
"It feels great to be in the last four", Djokovic said, via Wimbledon's Twitter account. Growing in confidence, Anderson dominated the fourth set winning 79% of his service points, having the better of the baseline exchanges, and seizing the break he earned through sustained aggression.
Not quite to the same extent as Anderson, but Djokovic too was forced to improve as his meeting with Nishikori raged on, particularly after the 24th seed showed a sturdier response to level after two sets. "Beating Roger Federer here at Wimbledon will be one I remember, especially in such a close match", Anderson said.
"Sometimes you don't feel good, and you try your best. I hope I can get a chance to fight for a trophy".
"I have already gotten tons of messages from support back home", Anderson said.
Eyeing a ninth title at The All England Club, Federer ultimately paid the price for being unable to convert any more than three of 12 break-point chances in the four-hour, 14-minute cliffhanger on Court One. Djokovic, a 12-time major champion, had not reached the final four of a Grand Slam event since the 2016 US Open.
Earlier on Centre Court, Novak Djokovic advanced to the Wimbledon semi-finals for the first time since 2015 with a 6-3, 3-6, 6-2, 6-2 victory against Japan's Kei Nishikori. Asked if losing to Anderson would spur him to return and make amends next year, Federer hinted he didn't plan to walk away just yet.
Djokovic, 31, will be playing in his 32nd Slam semifinal where he will face either world No 1 Rafael Nadal or fifth-seeded Juan Martin del Potro.
When he finished runner-up to Rafael Nadal at the US Open previous year, he was the first South African man in a Slam final since Curren at the 1984 Australian Open.
The Swiss refused to blame his surprise defeat on the decision to play the match on Court One rather than his customary Centre Court stage at the All England Club.