The clock is turned off during points, but it rarely rests when the ball is not in play. It starts from the moment players walk on court, counting down the one minute they are allowed between arriving at their chairs and attending the prematch coin toss. The clock then counts down five minutes for the warm-up, and one minute again between the end of the warm-up and the start of the match. Between games, it counts down from 30 seconds; during changeovers, it counts down 1 minute 30 seconds; and between sets, two minutes. It also displays the three minutes allotted for medical timeouts.

The first time the clock hits zero before a player begins the service motion, the player receives a warning. For every subsequent time it happens, the player loses a first serve. The returner is instructed to play at the pace set by the server.

Serve clocks have been part of tennis before. They were used in the qualifying rounds of last year’s U.S. Open, and in team tennis competitions like World TeamTennis. The ATP NextGen event last year in Milan also used a clock, and the “great success” there, Bradshaw said, gave his tour confidence it could work at regular tournaments.

For many players, the clock has taken some getting used to.

“At the beginning of the match, I was watching all the time,” David Goffin said after his first match in Washington. “Am I all right on time? Do I take more than 25 seconds or not? I was a little bit stressed about the clock since it was the first time, but by the end I didn’t watch anymore.”

Andrea Petkovic, who reached the semifinals in Washington, adjusted over the course of her four matches.

“Once I realized I’m doing fine — it’s always at a solid 19, 20 seconds when I start serving — it relaxed me actually,” she said.

John Isner, who gets more time violations than most players, welcomed the clock as an indicator of when to rush and when to relax.