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 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.