Roger Federer struggled into the Swiss Indoors semifinals on Friday after he eked out an error-filled 7-6 (7/1), 4-6, 6-4 defeat of Gilles Simon.
Federer has not hit top form all week in Basel and he needed more than two-and-a-half hours to hold off the troublesome French veteran.
The Swiss crowd favourite was never able to comfortably take command against an opponent whom he had defeated in their last six meetings.
It took a fightback in all three sets to come through for a place in a 14th semifinal here .
The 20-time Grand Slam champion will have to re-group quickly for the semis, where he will play the winner from Greek breakthrough player Stefanos Tsitsipas and Daniil Medvedev.
Both young guns have won tournament titles this autumn.
In the quarters, Federer found himself in 2-5 trouble after another sluggish start, with Simon quickly finding his range against the number three.
The Swiss had his chances to recover in the seventh game, but two break-back chances went for nought.
Federer started his slow recovery with a break for 4-5; he then saved set points to level at 6-all, winning a game in which he double-faulted twice in a row and saved a set point with an ace.
Federer came alive in the tiebreaker, racing to a 5-1 lead and earning the set after an hour of hard graft on a Simon backhand error.
Just as Federer looked to be settling into form after nearly 30 unforced errors in the opening set, more disruption ensued.
The 32nd-ranked Simon continued to make the match a struggle, with the pair trading four breaks in a row until Federer took a 4-3 lead.
But the margin lasted for only two games as the Swiss dropped serve for the fourth time to trail 4-5 as Simon struck a down-the-line winner.
The Frenchman levelled the sets at one apiece a game later.
The third set looked like going Federer's way, with the seed breaking in the second game, but losing serve in the seventh.
Simon found level-pegging for 4-all. But a quick match point opportunity two games later allowed Federer to close as his opponent sent a return over the baseline.
Earlier, second seed Alexander Zverev hammered a dozen aces to reach the final four with a 7-5, 6-3 defeat of Roberto Bautista Agut.
Nishikori scores
Japan's Kei Nishkori enhanced his prospects of appearing at the ATP Finals in London next month with a 6-3, 6-1 quarterfinal win over top seed Dominic Thiem in Vienna on Friday.