WIMBLEDON, England — Coco Gauff toed the line to serve, eyes focused, shoulders back, ready to go. It was a moment of peril in her semifinal mixed doubles match here on Wednesday. Break point. One game all, third set.

Gauff aimed a tight-spinning serve toward Matthew Ebden, her male opponent, and the point was on: a perfect display of what makes Gauff great at age 18, and what makes doubles an enduring favorite for Wimbledon fans.

Her teammate, Jack Sock, soon entered the mix, handling a…