Two days after ousting No.1 seed Coco Gauff in just 62 minutes, there was no letdown for the 18-year-old from Toronto.
After taking a back-and-forth - and error-filled - first set, Mboko was broken in a sluggish start to the second set but broke back in the fourth and sixth games to take a 4-2 advantage.
Mboko is the first Canadian to reach the WTA 1000 event's semi-finals since Bianca Andreescu's title run in 2019. She's also the youngest woman to reach the semis since Belinda Bencic's 2015 win in Toronto.
Mboko will face Elena Rybakina, who led Marta Kostyuk 6-1 2-1 when the Ukrainian was forced to stop playing because of an apparent arm injury.
Rybakina, the No.9 seed from Kazakhstan, converted three of her 10 break-point chances in the quarter-final matchup at IGA Stadium. The players shook hands at the 54-minute mark before the 24th-seeded Kostyuk exited the court in tears.
After the third game, Kostyu's trainers wrapped her forearm in medical tape.
In a breakthrough year, Mboko has surged from outside the top 300 to a career-high No.85. That number is projected to climb to at least No.55, according to WTA live rankings.
After two injury-plagued years, Mboko, who had shown promise as a junior, opened the season with a 22-match win streak and captured five titles on the lower-tier ITF Tour.
She then qualified for her first grand slam main draw at the French Open, reaching the third round, before stunning 25th-seeded Magdalena Frech in the first round at Wimbledon.
And the upsets keep coming.
In Montreal, Mboko has rattled off wins over 79th-ranked Australian Kimberly Birrell, No.23 seed Sofia Kenin, 39th-ranked Marie Bouzkova and Gauff, who is ranked No. 2.
Her latest victory boosted her record to 25-8 against higher-ranked players and 51-9 in all competitions.
In the other quarter-finals on Tuesday, No.6 seed Madison Keys - the highest-seeded player remaining - faces No.16 Clara Tauson, and former No.1-ranked Naomi Osaka meets No.10 seed Elina Svitolina.
The tournament final is on Thursday.
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