Denmark's top female tennis player reached the semi-finals in last year's edition but failed to replicate that form this time around.
Tauson actually got off to a good start by breaking Bartunkova's serve in the opening game. However, the Dane was broken twice later in the first set.
The second set began poorly for the Dane, who was broken twice in the first three games. Tauson then fought her way back to level the score at 3-3.
The Czech player responded by breaking the Dane's serve and went on to clinch the set and the victory. Before the tournament began, Tauson was ranked number 30th in the world.
Following the defeat in Toronto, she risks dropping about ten places in the rankings. This will affect her chances of being seeded at the US Open Grand Slam tournament, which begins in late August.
Only the top 32 players on the WTA world rankings are seeded at the US Open.
Tauson had a stellar 2025, finishing the year ranked number 12 in the world and securing several victories over top-10 players.
However, this season has been less successful for Tauson, marked by several defeats.
She has been plagued by injuries and revealed in May that she was suffering from two herniated discs. Since then, Tauson has struggled to regain her top form.