" Halfway through his strawberries [at breakfast], he [Marty] heard his parents' voices from the hallway. Their conversation was light and happy-sounding, the two of them having just come in from outside. / "Where were they, anyway?", Marty asked. / "Same as always," Dave replied. "Tennis at the club." / "Tennis? Dad and Mom don't play tennis." / "Then that explains why they've been club doubles champions for six years," Linda said archly. / "I can't believe it." / "Where have you been?" Linda demanded. / Marty was seriously considering telling her when his folks walked in the room. Their appearance was quite a shock to Marty. Both looked tanned and healthy in their tennis outfits, but the transformation was far deeper than that. George McFly radiated confidence and self-esteem while Lorraine was thin and dynamic-looking. "
—From Back to the Future by George Gipe (page 243)

Tennis was a sport that George and Lorraine McFly played in the improved 1985. They were the champions of their tennis club for the past six years by 1985.

Grays Sports Almanac contained every major sporting event from 1950-2000, including tennis.