Novak Djokovic collapsed to a three-set defeat by American Sam Querrey in the second round of the Paris Masters.
The Serb, who is guaranteed to finish the year as world number one, looked to be in total control when he took the first set without dropping a game, reports the BBC.
But world number 23 Querrey fought back to take the second on a tie-break and took a decisive break in the decider for a 0-6 7-6 (7-5) 6-4 victory.
There were also wins for David Ferrer, Juan Monaco and Janko Tipsarevic.
Spanish world number five Ferrer, who beat compatriot Marcel Granollers in straight sets, is now the highest seed left in the bottom half of the draw following Djokovic's defeat.
The second seed, who will replace Roger Federer at the top of the world rankings despite losing to Querrey, arrived on court in Paris wearing a Darth Vader mask to mark Halloween.
And Djokovic's jovial mood looked merited as he won the first eight games of the match. But Querrey, with his powerful forehand and a serve that yielded 18 aces, improved after finally making an impression on the scoreboard.
On his dreadful start, Querrey said: "I was embarrassed and wanted to walk off the court.
"But I finally held, then got a break, and I was on my way. I served terrific and I'm hoping to keep it up."
Novak Djokovic arrives on court wearing a Darth Vader mask
Querrey will play either 14th seed Milos Raonic of Canada or Frenchman Jeremy Chardy in the last 16.