A format change delivers big names, attention and crowds for mixed doubles. Expect it to stay—if the singles players keep signing on.

The U.S. Open's largest court was packed for the final three matches of the reimagined mixed doubles tournament Wednesday night, which was made up of mainly top singles players for the first time.

The semifinals of the new-look U.S. Open mixed doubles championship are set, with the title being decided tomorrow. The teams of Iga Świątek/Casper Ruud and Sara Errani/Andrea Vavassori advanced in the early wave, and they will be joined by Danielle Collins/Christian Harrison and Jessica Pegula/Jack Draper.