Defending champion Sabalenka - bidding to become the first woman to win back-to-back US Open titles since Serena Williams in 2014 - prevailed 7-5, 6-1 at the Arthur Ashe Stadium.
Sabalenka, though, did not have it all her own way, surrendering an early break in the first set to trail 3-2.
But the Belarusian ironed out the kinks in her game rapidly, breaking back to level at 3-3, and then breaking again to take the first set.
Sabalenka trailed 0-40 on her opening service game of the second set, but flipped a switch to recover and hold before racing into a 5-0 lead.
Masarova held to delay the inevitable, but Sabalenka duly closed out victory.
"Happy to get this win and happy to be in the second round. I feel like I didn't start my best in the first games, but then I found my rhythm," said Sabalenka, who will face unseeded Russian Polina Kudermetova in the second round.
