The world No 1 was made to work hard against Matthew Ebden of Australia in his last-eight clash at Halle, where he is hoping to retain his crown and win a 10th title.
The 36-year-old added a 19th consecutive grass-court win as he came out on top 6-2, 7-5.
Federer’s imperious run of form on his favourite surface saw him triumph at the Stuttgart Open last week as well as having won at Wimbledon for an eighth time 12 months ago.
But despite a three-month break as he once again skipped the clay-court season, the six matches he had played in nine days before facing Ebden, twice being taken to three sets, appeared to take its toll.
Especially after being taken to three sets in a match just shy of two hours against Benoit Paire the day before, requiring a decisive tiebreaker in the third set.
Federer must win in Halle to stay No 1 over rival Rafael Nadal going into the tour’s next Grand Slam at SW19 at the start of July.
But while the 20-time Grand Slam champion is the favourite to win at both grass-court tournaments, he will have raised some fears among fans by admitting he was not feeling fully sharp against Ebden.
Federer had the same schedule last year but played just one match in Stuttgart, losing to Tommy Haas to give him several days of rest before Halle and then Wimbledon.
He told ATP World Tour: “I think today I was a little bit tired so I had to push myself a little bit more and that’s why it was a bit harder to come up with good passing shots when I really needed it.
“Matt made life tough for me today but credit to him coming out and playing a really good quarter-final.
“I think it was another tough match. I’d never played Ebden before but I knew him quite well from practice.
“I think I was just very happy that in the bigger moments I actually served alright and I returned very well when I had to.
“His serve maybe let him down in the biggest moments and he had the break twice in the second set and should have brought it home somehow.
“But to break him three times in the second set I think is going to give me some confidence going into the next match.”
Federer suggested that while he is feeling the effects of an overload of matches in such a short period of time, he has it within him still to push for another Halle title.
He added to reporters post-match: “It didn’t come as easy as it did in other matches.
“But you can’t allow that to be frustrating. I can take away a lot of positives.
“Now that I’m in the semi-finals I can see the light at the end of the tunnel.
“You either lose in the semis or have two more matches, max. That allows me to find extra energy.”
Federer returns to court today where he will face American qualifier Denis Kudla in the Halle Open semi-finals (10.30am).
Should he reach the final and win, Federer will still get a week’s rest before his opening match at Wimbledon on Centre Court on Monday July 2.