Aryna Sabalenka had been here before: facing an American crowd under lights, with a lead, and feeling that lead start to slip away. Twelve months ago, she led Coco Gauff 1-0, winning the first set 6-2. Gauff won the next two sets and the title.

Sabalenka found herself in a similar scenario against Emma Navarro, serving for the match at 5-4 with a crowd she had mostly silenced finally screaming the house down on Arthur Ashe. This time, she got to a tiebreak from 5-6 down and got over the line.

For Karolina Muchova, still fresh off her return from wrist surgery, everything was going according to plan. She was routing American world No. 6 Jessica Pegula, and at 6-1, 2-0, 30-40 on Pegula’s serve, the Czech could see a Grand Slam final.

Tennis doesn’t work like that. After slinging Pegula wide with a forehand approach as she had done all night, Muchova came into the net to put the ball away. She didn’t control the ball, and it dropped long. That proved to be the match, even with 16 games to go.

Such were the small margins of the semifinals.