Roger Federer survived a surprise test at the Swiss Indoors on Friday before making the semi-finals at his home event for the 14th time with a hard-fought 4-6, 6-1, 6-3 win over Adrian Mannarino. The French seventh seed, who had dropped 10 straight sets in four losing matches against the world number two, shocked the home crowd at the St Jakobshalle by winning the first set. Mannarino broke to love for a 5-4 lead to quickly establish early superiority. But Federer, who lost just eight games in his first two matches, rose to the occasion, levelling at a set each with two service breaks.
In the third set, the Swiss top seed saved break points in the sixth game for 3-3 before breaking for a 4-3 lead with a backhand winner.
Federer, 36, completed his great escape one game later with a break, firing an untouchable passing shot on a first match point to take the win in just under one and three-quarter hours.
"Just because you've never lost a set, people expect it to keep happening," Federer said.
"But every day you feel different. It's hard to start out and play your absolute best next time.
"I had to dig and fight tonight. It was tough for me to get into it, he was solid in the first set and deserved to win it. But the reaction from me was really important."
The Swiss veteran fired 12 aces and advanced to a semi-final against third seed David Goffin who downed American Jack Sock 7-6 (8/6), 6-3.
Juan Martin del Potro struggled to dominate Roberto Bautista Agut on his way to a 6-2, 2-6, 6-4 win as the giant Argentine reached his third successive tour semi-final.
Del Potro, the fourth seed, is fighting for a place in the eight-man ATP Finals in London in just over a fortnight.
He came to Switzerland with a Stockholm title from last week and a Shanghai semi-final boosting his chances.
A third Basel title at the weekend at an event where Federer is looking for his eighth home trophy would send the 29-year-old Del Potro into the eighth and final position for the London showpiece.
He was a distant 47th in the race in late August and his efforts appeared to take their toll on Friday as he suffered a back niggle which required a visit from the trainer.
"I survived, I'm still standing," Del Potro joked. "I've been playing a lot of matches.
"I've had some problems in my body because I've been playing so many matches in a row.
"But I'm not putting my body at risk if I play tomorrow or next week (Paris Bercy). I speak a lot about fitness with my team. There is no risk to keep on playing."
"I will be ready for Saturday, it will be a challenge for me, I'm glad to reach the semi-finals of this event again."
Del Potro, now 17-3 since the start of the US Open, won the first set on Friday but found himself facing a challenge in the second as Spain's Bautista Agut, seeded sixth, broke twice to square the match.
In the deciding third, the Argentine, who beat Federer in the 2012 and 2013 Basel finals, traded service breaks with the Spaniard before winning the seven-miniute penultimate game with a break for 5-4.
Del Potro served out the match to move into a semi-final against second-seeded defending champion Marin Cilic, who struggled for three hours to beat 100th-ranked Hungarian qualifier Marton Fucsovics 7-6 (7/3), 5-7, 7-6 (7/4).