Tennis players use tennis balls during their match.
The referee is the one who begins the match (linesmen don't count)
There are six linesman in a match plus one umpire.
Two players on each side compete in a tennis doubles match.
After every point.
After about 3 sets of recreational (amateur) tennis, or 8 games of professional tennis.
There are 4 players on the court in a doubles match.
It's a tennis match, and a track meet.
Blaine Willenborg in his last professional tennis match lost in a doubles match during a somewhat of a comback against Vincent Spedia and his partner.
They drink Gatorade or water.
The match referee in badminton is called the umpire. There are 8 other linesmen for making line calls.