Tennis moves ahead with instant replay
The formula is somewhat similar to what the NFL is currently using.
Each player receives two (2) challenges per set.
If the challenged call is upheld, then the player loses one of their challenges.
If the challenged call is overruled, the player retains their challenge.
An extra challenge would be given for tie breaks, and challenges do not carry over from one set to the next set.
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Pretty logical, eh?
Note: Challenges are expected to take only 10 seconds per occurance.
The article continues with a discussion of challenges being used as a ploy in gamesmanship.