I hit the tennis drill jackpot recently and am mining it for all it is worth. Another great game inspired by Mike Barrell, evolve9: Luck of the Draw.
Create some cards with a variety of positive and negative tweaks to the usual rules:
Opponent gets only one serve
Alleys allowed (singles)
Only one serve
Use a different racquet
Use opposite hand
Wild Card - choose a second card; player decides who it should apply to
Use these cards to present challenges or opportunities to your players as they play out points. For example, have two players or teams play a tiebreaker. Before each point, have one of the teams draw a card. Alternate which team draws, or allow each to draw, or allow team that won last point to draw. Stay flexible and let your imagination guide you on the best use of the cards. Great way to train them to deal with positive and negative momentum changes that may occur during a match.
Update: suggested by my students - bring blank cards and a sharpie - they wanted in on the fun and wanted to create their own cards. Love it!