Sinner won the ATP Finals last year by beating American Taylor Fritz in the final.
Taylor Fritz beats Lorenzo Musetti at ATP Finals, Carlos Alcaraz again world number 1
Sinner and Auger-Aliassime are in the Björn Borg group with the German Alexander Zverev and the American Ben Shelton. Zverev won against Shelton in straight sets on Sunday evening.
In the first set it was an even match between Sinner and Auger-Aliassime, number 8 in the world rankings. Sinner did have a few break opportunities, but only struck on his third set point at 6-5.
Auger-Aliassime immediately lost his serve in the second set, causing Sinner to lead 3-0, after which the Canadian requested a medical timeout for treatment on his calf muscle. After a second break, Sinner was able to serve out the game and finish it with an ace on his first match point.