Karen Khachanov, a Russian tennis player of Armenian descent, defeated incoming world number one Novak Djokovic to win his first Masters 1000 title at the Paris Masters.
The 22-year-old beat Djokovic 7-5 6-4 in one hour and 37 minutes to end the Serb's 22-match unbeaten run, BBC reports.
Fourteen-time Grand Slam champion Djokovic, 31, will still overtake Rafael Nadal at the top of the world rankings on Monday.
"It means the world to me," said world number 18 Khachanov.
"It's one of the biggest titles in my career so far. I couldn't be happier to end the season like this, to win against Novak Djokovic, the number one in the world."
Khachanov will rise to world number 11 on Monday and becomes the first Russian since Nikolay Davydenko in 2009 to win a Masters 1000 title.
He hit 31 winners as he pulled off one of the shocks of the tennis year in his final tournament of the season.
Djokovic, who has been suffering from flu-like symptoms all week, added: "I don't feel too good today but I'd like to give respect to Karen for playing a great match today and he deserves to win.
"He deserves his trophy and I am sure we will see a lot of great matches from him in the future."