In terms of duration or number of games played?
If you're defining the longest match in terms of the number of games played, the 1985 US Open quarter-final between Steffi Graf and Pam Shriver is an obvious candidate.
1985 US Open Quarter-final
Graf d. Shriver 7-6 (7-4), 6-7 (3-7), 7-6 (7-4)
Best left-right combination by a German (and that includes Max Schmeling): Steffi Graf. All she did in 1987 was knock Navratilova out of #1 and try to knock Evert out of the sport. (Mike Lupica in "The Best and Worst of Tennis in 1987", World Tennis)
"A couple of years ago, we nicknamed Steffi Graf's forehand 'Jaws'. And that music would go perfectly when she starts running in to the net, swarming on that little ball." (JoAnne Russell, during the 1988 Wimbledon final between Graf and Navratilova)