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.”
Alcaraz and Ruud are joined in the John Newcombe Group by German Alexander Zverev – a two-time ATP Finals champion – and Russian Andrey Rublev, who play in Monday’s evening session from 19:30 GMT.
