According to Sky Sports commentator Mark Petchey, Federer was sick coming into the Swiss Indoors.
And in another big clue he could be struggling, the Swiss legend admits he might not play at the Paris Masters next week.
It seemingly hasn’t affected him on the court – he’s crushed Frances Tiafoe and Benoit Paire in rounds one and two – but Petchey believes he’s not feeling his best.
Petchey said: “Federer looked a little under the weather against Paire. He has been sick coming into this tournament and didn’t look 100 per cent.
“He played great against Tiafoe in the first round, he really did.
“It was a very dangerous first-round match for him and he gave Tiafoe the respect and due care and attention.
“He played decently against Paire but he didn’t play great, the Frenchman wasn’t at his best.
“He missed a lot of backhands as well as forehands.”
Federer is 1,960 points behind Nadal in the ATP rankings going into the last few weeks of the season.
A tournament win in Basel would reduce the deficit to 1,460 points with the Paris Masters up next.
The world number two will face Belgium’s David Goffin later today in the semi-finals.
Federer has been given a bye at next week’s Paris Masters, and he could decide to skip the tournament to focus on the season-ending World Tour Finals in London.
FEDERER PULLS OFF WONDERSHOT
This is the Roger Federer shot which is being dubbed the best of the season.
The 19-time Grand Slam champion needs to win the Swiss Indoors this week to keep his chances alive of catching Rafael Nadal at the top of the rankings.
But for large periods of his quarter-final clash against Adrian Mannarino, Federer was staring down the barrel.
Mannarino won the opening set 6-4 to heap pressure on Federer in front of his hometown fans in Basel.
Federer came back to win the second before the Frenchman carved out two break points in the sixth game of the deciding set.
Federer saved the first but missed his first serve at 30-40 down – before this (above) happened.
The Swiss star went on to win the match 4-6 6-1 6-3.