Spain's Carlos Alcaraz saw his hopes of finishing the year as the world number one damaged by a shock opening loss to Roman Safiullin in the Paris Masters.
World number two Alcaraz, who received a bye in the first round, lost 6-3 6-4 to 45th-ranked Safiullin.
The Russian qualifier played superbly throughout and Alcaraz had few answers in a below-par performance.
It leaves Alcaraz with a tough task to overhaul Novak Djokovic at the top of the rankings before the season ends.
Alcaraz, who won Wimbledon in July for the second Grand Slam title of his career, pulled out of Basel last week with injury and was playing for the first time since losing in the Shanghai last 16 on 11 October.