Casper Ruud stunned an out-of-sorts Carlos Alcaraz to begin his ATP Finals campaign with a straight-set victory in Turin.
The Norwegian sixth seed won 6-1 7-5 for only his third victory in 12 matches.
In a below-par performance, French Open and Wimbledon champion Alcaraz hit 34 unforced errors and fell to his first defeat by Ruud in five meetings.
"It's only one match but it's definitely one of the best wins of the season for me," said Ruud, who suggested Spanish third seed Alcaraz was struggling with a cold.
"I feel very happy.