The Briton showed resilience in the early stages by staving off five break points in her opening service game.
She then wrapped up the opening set in style by winning five successive games.
After exchanging a couple of breaks in the second set, Raducanu – who also beat Ruse in Auckland in 2024 – found the crucial break to lead 5-4 and served out for the match.
Since losing against Aryna Sabalenka at Wimbledon earlier in July, Raducanu has bounced back strongly with four victories from five appearances on hard courts.
If she gets past Stearns in Montreal, American fifth seed and 2025 Wimbledon finalist Amanda Anisimova could be waiting in the third round.
Boulter narrowly avoided being on the end of a bagel against Zarazua, saving two set points to get to 5-1, but the Mexican swiftly wrapped up the opener when back on serve.
The 28-year-old put up greater resistance in the second but never managed to get a foothold in the match as she crashed out.