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 his 50th victory of the season, yet only his third in 12 matches.
Later, German second seed Alexander Zverev – a two-time ATP Finals champion – defeated Russian Andrey Rublev 6-4 6-4.
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. I think we’ve all seen Carlos play better tennis than he did today but I took care of my chances.
“I’ve not been oozing with confidence in the last weeks or months so it was a great win for me and hopefully I can build on it.”
After fending off two break points in the opening game, Ruud was the first to break after a number of wayward shots from his opponent.
Though Alcaraz continued to struggle to find the court, he had the opportunity to level in the next game but at break point saw his forehand deflect out off the net cord.
Two further chances followed but Ruud withstood the danger and was rewarded for his resolve when Alcaraz again placed a forehand out of bounds for the double break.
Ruud served out the set to love with an ace, only the second set the 25-year-old had taken from Alcaraz – who hit 18 unforced errors and landed less than 50% of his first serves in the opener.
But the Spaniard reset for the second set, dropping just a single point across his first three service games before breaking for the first time.
He looked to be cruising towards levelling the match but Ruud fought back – earning himself two break-back points with a lob on to the baseline, but needing only one as Alcaraz netted a backhand.
As Alcaraz’s level continued to slip, a rejuvenated Ruud – who had been just two points from losing the set – broke again before serving out the win with an ace on his third match point, capping a run of five straight games.
“It sounds like an excuse and I don’t want to take anything away from what Casper did to win the match,” said Alcaraz, 21.
“Before coming here, I was unwell at home. When I got to Turin, practice was going well; I could handle long rallies, competing and playing. And I hoped things would get better as the days went by.
“I don’t know if it was because of the nerves of the first match, but I didn’t feel completely well. My stomach limited my game a lot and made me feel uncomfortable.”
Ruthless Zverev takes his chances to beat Rublev
Not since the group stage of last year’s ATP Finals had Zverev and Rublev met on court, with the German winning in straight sets on that occasion.
In the second match of the John Newcombe Group, little could separate the pair in Turin and after six games of a tight opening set, eighth seed Rublev had yet to drop a point on serve while Zverev had lost only two.
But Zverev got the decisive break in the next game when Rublev played a forehand too deep at 15-40 down.
The second set was equally evenly matched but Zverev moved ahead in exactly the same manner, drawing an error off the front of Rublev’s racquet on his first of two break points.
That put him 5-4 up and he served out the match with an ace, extending his record over Rublev to 7-3.
In the Ilie Nastase Group on Tuesday, Rublev’s compatriot Daniil Medvedev faces Australian Alex de Minaur from 13:00 GMT.
Home favourite Jannik Sinner – who received his year-end world number one trophy at the start of Monday’s evening session – takes on American Taylor Fritz from 19:30.
