Hopes were good for Tomljanovic on Wednesday following her impressive victory over Veronika Kudermetova the previous day.
But 21-year-old Swiatek, who is enjoying a stellar year, broke twice in the opening set on her way to a 6-1 6-2 victory.
Although Tomljanovic, the world No.72, broke twice herself in the second set, she could not hold her own serve even once as the Pole moved through serenely into the last 16.
Playing her first match on hard courts in four months, Swiatek extended her unbeaten run to 19 matches in just her second-ever main draw played in Canada. In her 2019 debut, as a teenage qualifier, she reached the third round before losing to then-No.2 seed Naomi Osaka.
"Both of the matches that I played here in main draw against Caroline Wozniacki and against Naomi was like a breakthrough for me," Swiatek said.
"It really gave me extra motivation. Because even though I lost this match against Naomi I remember I was just happy that I could play such a tight match against her. She was world No.1 back then so it was a big deal for me for sure."
Another young gun in American Coco Gauff outlasted Wimbledon champion Elena Rybakina 6-4 6-7 (10-8) 7-6 (7-3) in a marathon clash, while Czech Karolina Pliskova outclassed Amanda Anisimova 6-1 6-1.
Gauff, the 18-year-old French Open finalist, cruised through the first set with four aces, breaking Rybakina in the seventh game, and appeared to have the match firmly in hand.
But Rybakina refused to go down without a fight, fending off four match points in the second-set tiebreaker before taking it.
A rattled Gauff served seven double faults in the third set but dug deep to win five straight points and clinch the third-set tiebreaker.
Former world No.1 Pliskova had no such issues against American Anisimova, breaking her in the fourth game of the first set before running away with the match.
Pliskova stormed home in 53 minutes just a few days after Anisimova bounced her out in the second round of the Silicon Valley Classic and next faces Greek powerhouse Maria Sakkari.
Elsewhere, second seed Anett Kontaveit of Estonia was upset 6-4 6-4 by Switzerland's Jil Teichmann who now takes on two-time grand slam champion Simona Halep in the round of 16 after the Romanian eased past China's Zhang Shuai 6-4 6-2..
Defending champion Camila Giorgia of Italy was a 6-3 7-5 winner over Belgian Elise Mertens while seventh seed Jessica Pegula won the all-American tie against Asia Muhammad 6-2 7-5.
With agencies.
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