A 25-second serve clock will be used in main-draw matches at the U.S. Open and about a half-dozen other hard-court tournaments leading up to the year's last Grand Slam tournament.

The U.S. Tennis Association, ATP and WTA announced Wednesday that those events also will enforce pre-match time limits: Players will have one minute from walking on court until the coin toss, five minutes for warming up and a minute from the end of the warmup until the start of play. Players who don't abide by those limits will face fines.

Clocks will be visible on court.

If the serve clock runs out, the chair umpire will issue a time violation. Chair umpires will be able to pause the clock at their discretion.

The U.S. Open is the first Grand Slam tournament to use the serve and warmup clocks.

They also will be used at Washington, San Jose, Montreal, Toronto, Cincinnati, New Haven and Winston-Salem.