Felix Auger-Aliassime put a "weird start" aside to beat Ben Shelton and register his first victory of this year's ATP Finals.
Canada's Auger-Aliassime came back from a mixed first set to beat the big-serving American 4-6 7-6 (9-7) 7-5 in Turin.
The 25-year-old was increasingly impressive on serve and showed no signs of the calf problem that hampered him in his opening group-match loss to Jannik Sinner.
Eighth seed Auger-Aliassime's victory means Italy's Sinner will qualify for the semi-finals if he beats Alexander Zverev later on Wednesday.
Germany's Zverev will only advance if he beats Sinner in straight sets.