Tsitsipas turned 23 on Friday (AEST) and marked the occasion with a 6-3 6-2 win over Karen Khachanov.
The Greek star was presented with a cake by tournament officials after his win, as the crowd serenaded him with a chant of 'happy birthday'.
It marks a significant improvement on when Tsitsipas turned 20. On that occasion, he lost the 2018 final of this event to Rafael Nadal.
Next up for Tsitsipas is Casper Ruud, who poked fun at his critics after he eased into the quarter-finals.
Ruud, the world No.12, won three titles on clay courts in July and has now continued that form in the ATP Masters 1000 event.
Playing on a hard court has proved no challenge for the in-form Norwegian, who wrote 'Hard Courts' with a smile underneath on the camera after his 6-4 6-3 victory over Dusan Lajovic on Friday (AEST).
"I have seen a lot of people questioning my ability to play on hard court, so I just thought it was funny to write a little note," Ruud said.
"I think it's fun to play on hard courts. Even though I (have achieved) most of my best results on clay, my best slam result is from this year’s Australian Open (where he reached the Round of 16), so I think it's a surface that also can suit my game well even though this year has been mainly clay.
“I think I'm motivated to come back to the hard courts. I have had two very good matches here. It's a great start for me to the hard-court swing."
Ruud, in the hunt for his fifth title of 2021, has won the only previous meeting between the duo.
"[I am] looking forward to that battle," Tsitsipas said. "We have played each other in the past. Not an easy guy to play against. Looking for a change, looking for a switch-up this time."
However, there was no place in the last eight for world No.7 and Olympic gold medallist Andrey Rublev, who went down 7-5 7-6 (7-5) to American John Isner.
Fresh from a triumph in Atlanta, 36-year-old Isner has looked sharp all week and has now won eight successive matches.
It brings up a third quarter-final appearance for the world No.30 in Canada, and he will play Frenchman Gael Monfils, the No.11 seed, after he got past lucky loser Francis Tiafoe 6-1 7-6 (7-2). Monfils will be competing in his first ATP Tour quarter-final in 18 months.
No.1 seed Daniil Medvedev proved too good for Australian qualifier James Duckworth, winning 6-2 6-4 in one hour and eight minutes.
Medvedev was in ominous form on the hard court, sending down 10-1 aces and winning 27 of 32 points on his first serve.
The world No.2, from Russia, will play No.7 seed Hubert Hurkacz who fought back to win 6-7 (7-9) 6-4 6-4 over Nicolas Basilashvili.
No.8 seed Diego Schwartzman lost in three sets to No.10 seed Roberto Bautista Agut after the Spaniard saved three match points.
Bautista Agut will next play Reilly Opelka who came from behind, and also saved a match point, to win over Lloyd Harris in three sets.