For nearly two hours on Sunday afternoon, everything had gone so smoothly and serendipitously for Jannik Sinner.

He had broken his opponent, Taylor Fritz, in the first game. That had given him an early lead, and eased any jitters he may have felt about playing his first US Open final, against an American, in front of thousands of people who would be rooting hard, and loudly, for him to lose.

In the second set, Fritz found his footing on his serve and began to hold easily. But Sinner managed to break him at the most convenient moment, with the American serving at 4-5. Suddenly, a contest that had featured plenty of high-velocity rallies, and that had looked as if could turn competitive, had turned into a rout instead. Despite making just 51 percent of his first serves, Sinner hadn’t been broken. He was defending Fritz’s best blasts from the baseline, and sending back harder, heavier, more precise responses. The Italian was keeping the Arthur Ashe Stadium crowd, which was itching to explode, at a low, frustrated murmur.

Sinner’s enchanted ride continued into the fifth game of the third set, when he reached double break point on Fritz’s serve at 2-3. He must have caught a glimpse of the finish line in that moment, and tightened up a bit at the sight, because he became tentative for the first time. He shanked a return, pulled a forehand wide, and watched as Fritz belted a 100-m.p.h. forehand to hold. The fans finally had a chance to get noisy, and the commotion only grew in the next game, when Fritz broke Sinner’s serve with a perfect lob and another triple-digit forehand.