Juan Martin del Potro struggled to dominate Roberto Bautista Agut on his way to a 6-2, 2-6, 6-4 win at the Swiss Indoors on Friday 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 Roger 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.
Del Porto, 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 either second-seeded defending champion Marin Cilic or Hungarian Marton Fucsovics.
Federer was aiming for the final four later Friday against France's seventh seed Adrian Mannarino, against whom he has never lost a set in four matches.