Federer suffered a shock defeat to Kei Nishikori in his opening group match and results elsewhere have since gone against the Swiss star.
Nishikori was thrashed by Kevin Anderson earlier today meaning Federer must win to keep his semi-final hopes alive.
But the world No 3 skipped practice yesterday because he feels the grass courts at Queen’s do not replicate the ATP Finals hard court.
And he also had a moan about the court speed following his defeat to Nishikori.
“I think it plays different or it’s definitely slower than I think the last three tournaments that I’ve played,” Federer said.
“So I think everybody’s making a minor adjustment, you know. So am I.
“I’ve been feeling, I mean, fine. It’s just that practice has been a bit all over the place. Practised in Queen’s, practised on the outside courts here, then centre as well.
“So it’s not always exactly the same conditions. Overall I thought I’m hitting the ball okay.”
Roger Federer vs Dominic Thiem LIVE – 8pm start
7.30pm: Good evening and welcome to the O2 Arena where Federer is about to take on Thiem in his second round robin match.
Federer knows he must secure a win to keep his hopes of progressing out of the group alive.
But the Swiss star has a negative record against Thiem, winning just one of their three previous meetings.
However, the pair have not played each other since 2016 so it will be interesting to see how their styles match up this time around.