Paris: World number two Carlos Alcaraz crashed to a 6-3 6-4 loss to Russia's Roman Safiullin in the second round at the Paris Masters on Tuesday.
The Wimbledon champion looked off the pace as he made his return to action after withdrawing from the Swiss Indoors championships last week due to injury, losing the first set despite earning an early break.
The 20-year-old Spaniard seemed to recover as he again broke the feisty 45th-ranked Saffiullin's serve early in the second set to take a 3-1 advantage.
But the 26-year-old Russian kept his foot on the gas to earn the biggest win of his career.
The result was another blow to Alcaraz's hopes of gaining ground on Novak Djokovic in the ATP rankings, with the top-ranked Serbian next in action on Wednesday against Argentina's Tomas Etcheverry.
Ninth seed Taylor Fritz's hopes of an ATP Finals berth diminished as the American withdrew before his second round match against Daniel Altmaier with an abdominal injury.
The world number 10 will be out of the season-ending tournament should Denmark's sixth seed Holger Rune beat Dominic Thiem in his second round match.
German 10th seed Alexander Zverev rallied from a sloppy start to beat Hungary's Marton Fucsovics 4-6 7-5 6-4, moving closer to securing his spot at the ATP Finals in Turin.
Poland's Hubert Hurkacz, the 11th seed, also kept alive his Finals hopes by earning a hard-fought 6-3 6-7(6) 6-3 win over American Sebastian Korda.
Argentine Francisco Cerundolo saved a match point and bounced back from a two-game deficit in the third set to beat home favourite Gael Monfils 4-6 7-6(2) 7-5.
Russia's 16th seed Karen Khachanov advanced to the third round past Serbia's Laslo Djere 6-4 7-5, while fellow Russian Andrey Rublev had little problem beating Japanese Yoshihito Nishioka 6-4 6-3.