The second-seeded Greek, who won his first Masters 1000 title at the Monte Carlo Rolex Masters last week, needed just 83 minutes to defeat the Canadian and improve to 25-5 on the season.
Tsitsipas extends winning streak with comfortable victory
Auger-Aliassime made the stronger start as he held comfortably and had three break points on Tsitsipas' first service game, but the Greek saved them all. His next service game went to deuce, but he again prevailed.
Tsitsipas then made his move in the fifth game as he broke Auger-Aliassime to love after the Canadian dumped a forehand into the net. The Greek claimed the opening set after the tenth seed struck a forehand long.
A finalist in 2019 here, Tsitsipas came through another deuce game in the third game of the second set and broke a game later after another error from Auger-Aliassime to go in front 3-1. Leading 5-3, the Greek slid an ace out wide to reach the semifinals in the Catalan for the second time.
"I think my hard efforts paid off", he said after the match. "I wouldn't say it was the best start, but I managed to win my serve from 0-40 down in the [second game]. That got me into the match and that helped me break him in the end."