Since 2005, the Hawk-Eye electronic challenge system has been the most widely used replay method in tennis. It was the only system approved by the various tennis federations for over a decade.
But in 2017, FOXTENN introduced its own “Real Bounce” line calling system.
Instead of an animated projection, FOXTENN uses slow-motion cameras to capture the bounce of the ball. It was used this past week in Acapulco, where some players weren't having it.
Reilly Opelka tweeted this about the system: