Williams has only played the Rogers Cup in Montréal once before, making the final back in 2000, but on Wednesday night it looked like she had never left, storming past Stosur - a player who had beaten her three times before, including in the 2011 US Open final - handily in straight sets, 6-0, 6-2.
The World No.1 then wowed the crowd by giving her interview to the French broadcaster - in French.
"It feels great to be here," she said in Montréal's native tongue. "I thought I played well tonight. She's won a Grand Slam title and she's a really good player, so it was important for me to play well.
"I love it here and I want to thank everyone for coming out tonight."
Though she has never won in Montréal, Williams has done very, very well at the Rogers Cup in her career, winning the Premier-level event in Toronto three times - 2001, 2011 and 2013. And she comes in off of some major momentum, winning her WTA-leading fourth title of the year in Stanford last week.
Next up is No.15 seed Lucie Safarova, who beat Magdalena Rybarikova earlier on, 6-4, 6-2. Williams has beaten Safarova in all six of their previous meetings, though some have been very competitive.
It was mostly an upset-free day session on Wednesday, with No.4 seed Maria Sharapova, No.6 seed Angelique Kerber, No.7 seed Jelena Jankovic and No.11 seed Caroline Wozniacki all winning through, with Jankovic having an especially tough day, edging Sloane Stephens, 6-7(7), 6-4. 7-6(4). Jankovic won just two more points than Stephens in the three-hour-and-15-minute marathon - 136 to 134.
The only upset saw British qualifier and WTA Rising Star Heather Watson win a thriller against No.10 seed Dominika Cibulkova, 6-2, 6-7(3), 7-6(5), originally leading 6-2, 4-0 with a point for 6-2, 5-0, and almost letting a 5-2 lead go in the third set as well before finally closing it out in a third set breaker.
Watson's win over the World No.12 was the equal-best win of her career in terms of ranking, having taken out a No.12-ranked Flavia Pennetta on her home turf in Eastbourne earlier in the season.
The night session saw No.2 seed Petra Kvitova beat Casey Dellacqua, 6-3, 6-2, and Venus Williams beat Kazakhstani qualifier Yulia Putintseva, 6-3, 6-2, but No.9 seed Ana Ivanovic go out in a match that finished after 12:30 at night to CoCo Vandeweghe, 6-7(7), 7-6(7), 6-4. Ivanovic led 3-0 in the third set before falling to the American qualifier - and WTA Rising Star - after two hours and 54 minutes.
Happy with the way I played tonight! Thanks to fans for staying after the rain. Now talking, stretching and eating :) pic.twitter.com/soBdokOZly