Ostapenko's next opponent will be revealed after Tuesday's match between World No.10 Belarusian Aryna Sabalenka and Russia's Anastasia Pavlyuchenkova.
This is Ostapenko's first victory at the Rogers Cup, having previously failed to go past the first round in three Canadian Open tournaments held in Toronto and Montreal. Ostapenko is currently ranked World No. 82, while her opponent Garcia is No. 22.
"It was a tough match. This season in general is very tough for me, so every win I get is very good for me and good for my confidence. Obviously, I want to get it back and be back on top," Ostapenko said.
On Tuesday evening (EEST), Latvia's Anastasija Sevastova (No.11) will play China's Shuai Zhang (No.35) in her first round at this year's tournament.
The Canadian Open is the third oldest tournament in all of tennis, behind Wimbledon and the US Open.