Basel - World number six Alexander Zverev lost a 7-6(9/7), 6-4 battle at the first round of the ATP Swiss Indoors to Taylor Fritz on Tuesday as the American hit 14 aces.
The defeat for the German to the 21-year-old came barely a week after losing in the Shanghai Masters final to Daniil Medvedev.
With Zverev next in the queue to qualify for November's eight-man ATP Finals in London - two spots remain open - the seventh-ranked player has his work cut out to make it into the year-ending showpiece.
Zverev, a semi-finalist last year in Basel, fought in the opening set to take it into a tiebreaker but was overwhelmed in the second by his American opponent in 86 minutes.
Fritz, ranked 31st and a winner at Eastbourne in June, saved both break points against his serve.
Fabio Fognini picked up his London qualifying pace with a 6-2, 6-4 defeat of Australian Alexei Popyrin.
The Italian winner reached the second round in 78 minutes to reinforce his year-end bid.
The Monte Carlo Masters champion stands 11th in the chase for one of the remaining eight spots in the field and is duelling with compatriot Matteo Berrettini.
Fognini said he is not particularly bothered about the race outcome.
"I'm 11th and that is still far away. If I qualify - fine.
"If not, then I can spend more time with the family, that's the most important thing anyway.
"I'm not a huge fan of playing all the big servers of today, for me it's not tennis. But I'm glad to still be playing at 33," he added.
Moldovan Radu Albot will line up in the second round against nine-time champion Roger Federer on Wednesday after a 2-6, 6-3, 6-4 comeback over Serb Dusan Lajovic.
The US' Riley Opelka hammered 27 aces in his 7-6(7/5), 7-6 (12/10) defeat of Chile's Christian Garin and will face either sixth seed David Goffin or Marin Cilic.