Casper Ruud dismantles error-strewn Carlos Alcaraz in ATP Finals opener
The round-robin format of the eight-man event means he could still crown a standout year in style, but the third seed has left himself with little margin for error in his remaining group matches and will need to find a higher level.
World number seven Ruud, who had lost all four of his previous meetings against Alcaraz, saved break points in the opening game but dominated the first set from then on as surprising errors flowed from the Spaniard's racket.
Alcaraz responded positively to move 5-2 ahead in the second set, breaking serve with a deft volley, and was two points from levelling the match only to capitulate.
He dropped serve at 5-3 and looked forlorn as Norwegian Ruud roared to victory, winning the last five games.
Alcaraz, who won the French Open and Wimbledon titles this year, is hoping to become the first Spaniard to win the prestigious year-ending title since Alex Corretja in 1998.
Ruud failed to convert his first two match points but closed it out at the third attempt to put himself in a strong position in the John Newcombe group.
"This is one of the best wins of the season for me," Ruud said in a courtside interview.
"I feel happy of course. I've seen Carlos play better tennis than he played today but I still had to take my chances."
The Ilie Nastase group began on Sunday with world number one Jannik Sinner beating debutant Alex De Minaur and American Taylor Fritz overcoming Russian Daniil Medvedev.
While Alcaraz did claim the Beijing title in September his form since winning Wimbledon has been patchy including a second-round exit at the US Open.
He made 34 unforced errors against Ruud with his usually destructive forehand misfiring and most of the drop shots he attempted either finding the net or being dispatched easily by the grateful Norwegian.
Ruud, who reached the final in 2022 when he lost to Novak Djokovic, said Alcaraz had been suffering with a cold in Turin.
"I knew he was maybe dealing with a cold, I've seen him snuffling around in the player area with tissues so that's a sign maybe he's not 100% physically," he said.
"That's sad for him but it's also part of the game and I knew it coming and wanted to make him play rallies. I was able to turn around the second set with some good tennis at the end."
In the evening match at the Inalpi Arena, twice former champion Zverev racked up a Tour-leading 67th match win of the season in clinical fashion with a single break of serve in each set enough to see off Rublev.
"It was a very solid match form my side," world number two Zverev said. "I used my chances well."
