Watched on by a home crowd packed with celebrities and sporting stars, Fritz got off to a shaky start when he dropped serve in the first game.
The 26-year-old recovered well to win three games in a row, but he continued to falter on serve and Sinner capitalised to take the opener and subdue the expectant mood on Arthur Ashe Stadium.
Refusing to dwell on the first set, Fritz improved and made the second a closer affair, losing just two points in four service games.
However, Sinner underlined his status as world number one, upping the ante when it mattered most to strike the crucial blow as Fritz served to stay in the set at 5-4 down.
Having failed to make the most of three break point opportunities early in the third set, Fritz brought the crowd to life with two brilliant winners to break for a 4-3 lead and keep his final chances alive.
But Sinner soon quashed those dreams, breaking back as Fritz attempted to serve for the set and doing the same two games later to wrap up the win after two hours and 16 minutes.
More to follow.