Jannik Sinner began his ATP Finals title defence by sweeping aside a physically hampered Felix Auger-Aliassime in front of a passionate Turin crowd.
Italian second seed Sinner was imperious on serve in a 7-5, 6-1 win over the Canadian.
Eighth seed Auger-Aliassime was brilliantly aggressive in a competitive first set and was two points away from forcing a tie-break before tweaking his left calf.
The 25-year-old called for the physiotherapist twice in the second set, where he won just one game.
Victory puts Sinner top of the Bjorn Borg Group as he bids to secure the year-end world number one ranking.
Sinner has not lost to a top-10 player other than Carlos Alcaraz since being beaten by Andrey Rublev in August 2024 – at a tournament where Sinner played two matches in one day.
Earlier, Taylor Fritz shone on serve to beat Lorenzo Musetti 6-3, 6-4 in the Jimmy Connors Group.
Sinner has been the dominant force on indoor hard courts, with this marking his 27th consecutive victory on the surface.
His serve was superb in a high-quality first set. He won 24 of 27 first-serve points, did not lose a single point behind his first serve, and did not face a break point.
Auger-Aliassime took a set off Sinner during his run to the US Open final and hit hard from the off, rushing Sinner and occasionally matching him shot-for-shot.
But the injury at 6-5 30-0 up had an immediate impact, with Auger-Aliassime struggling to move and sending his shots long as he tried to hit his way out of trouble.
Sinner, who was taken to deuce just once in 10 service games, did not let up and sealed victory with an ace before being serenaded by his home crowd.
He and rival Alcaraz are in contention for the year-end number one ranking in Turin.
Sinner must defend his title – and hope Alcaraz loses a group match and does not reach the final – to retain the top ranking.
Alcaraz, in the opposite group to Sinner, began his campaign with a straight-set win over Alex de Minaur on Sunday.
American world number six Fritz was also impressive in his victory over Italy’s Musetti.
Fritz, runner-up to Sinner last year, dropped just three points on serve in the second set – and two of those were in the final game.
Ninth seed Musetti only had his place at the season-ending event confirmed on Sunday, when Novak Djokovic withdrew after beating him in the Athens Open final.
He had four early break opportunities against Fritz but could not take them before fatigue set in, with seven of his nine service games going to deuce.
“I was a little shaky at the start and I was letting him dictate a little too much,” Fritz, 28, told Sky Sports.
“It was a bit nervy for me, which is expected. I felt I was able to loosen up, and I just started playing a lot better.”
In the doubles, Britons Joe Salisbury and Neal Skupski began their campaign with a 6-3, 7-5 win over Salvadoran Marcelo Arevalo and Croatian Mate Pavic.
No all-British pair has ever won the season-ending title, with Salisbury and American partner Rajeev Ram lifting it in 2022 and 2023.
Britain’s Henry Patten and Finnish partner Harri Heliovaara also won their opening match, beating American pair Christian Harrison and Evan King 6-4, 6-4.
