Norway's Casper Ruud eased to a straight-sets victory over Carlos Alcaraz on Monday as the Spanish star's bid for a maiden ATP Finals title got off to a poor start.
Ruud romped through the opening set before fighting back from 5-2 behind in the second to win 6-1, 7-5 in Turin.
World No 3 Alcaraz, who won the French Open and Wimbledon titles earlier this year, made 34 unforced errors in an uncharacteristically errant display.
It was Ruud's first victory over Alcaraz in their five career meetings.
"I had two good times in Turin before... This part of the season has not been good for me but hopefully I've saved some wins for this week and I've started well," said 2022 runner-up Ruud, who had suffered seven defeats in eight matches before this tournament.
Ruud will head into his second match in the John Newcombe group against Andrey Rublev on Wednesday already well-placed to reach the semifinals for the third time in as many appearances at the year-end event.
Alcaraz could be eliminated if he loses to second-ranked Alexander Zverev, who faces Rublev in his opener later on Monday.
Third seed Alcaraz quickly slipped 4-1 down, giving up his serve in the fourth game before seeing three break points come and go in the next.
Ruud stretched a double-break in front as Alcaraz fired a loose forehand wide and wrapped up the opening set in just 36 minutes with a hold to love.
The four-time Grand Slam champion improved early in the second set, though, bringing up his sixth break point of the match with a trademark forehand winner.
He took the opportunity with a delicate volley and then moved into a 5-2 lead with a clinical hold.
But Ruud dug deep to force Alcaraz to serve for the set before breaking back en route to levelling the set.
The errors suddenly started to flow again from Alcaraz's racquet and a wild forehand followed by an excellent Ruud return gave the sixth seed the chance to serve for the match.
Ruud closed it out in style on his third match point with an ace, completing a run of five straight games.