The second-seeded Pole fell 6-7, 3-6 to the 16th-seeded Tauson, who earned her first career win over Świątek.
In their previous encounter last month, Świątek defeated Tauson 6-4, 6-1 in the fourth round at Wimbledon, en route to capturing her maiden title in London and sixth career Grand Slam trophy.
Świątek, 24, is coached by Belgian Wim Fissette.
Meanwhile, Japan’s Naomi Osaka—now working with Świątek’s former coach Tomasz Wiktorowski—is enjoying strong results in Canada, Polish state news agency PAP reported.
On Sunday, she swept Latvia’s Anastasija Sevastova 6-1, 6-0 after earlier eliminating Poland’s Magda Linette in the second round.
Osaka was previously coached by Fissette as well.
Another Pole, Magdalena Fręch, also competed in this year’s Canadian Open but exited in the second round.
All three Poles—Świątek, Linette and Fręch—had a bye in the first round.
The Canadian Open is a joint women's and men's hard-court tournament held annually in Montreal and Toronto, with the host cities alternating each year.
Świątek skipped last year's event in Toronto due to recovery following the Paris Olympics and has yet to win the tournament.
In addition to her Wimbledon crown, Świątek holds four French Open titles and one US Open title from 2022.
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Source: PAP