She has often hit the high notes in early rounds and then run into a mental block, an opponent on a roll or a combination of the two when she was on the verge of winning her 24th Grand Slam singles title and tying Margaret Court’s record.

The latest of several setbacks: this year’s Wimbledon final against Simona Halep.

Williams, 37, had overwhelmed Barbora Strycova in the semifinals, 6-1, 6-2, only to turn shaky and be routed, 6-2, 6-2, by Halep, who was in a clean-hitting state of grace.

Elina Svitolina, Williams’s opponent in Thursday’s semifinals, is a world-class counterpuncher, as well. She is among the quickest and best defenders on tour and has navigated and hustled her way through a treacherous patch of the draw to reach her first U.S. Open semifinal less than two months after reaching the same stage at Wimbledon.

Seeded No. 5, she has defeated power baseliners like Venus Williams, Dayana Yastremska, Madison Keys and Johanna Konta without dropping a set.

“I played some big hitters in this tournament, a lot,” Svitolina said. “I have to just react quickly with my feet and with my shots, as well. Then when I have the opportunity, go for it.”