There was a rally in a match at richmond in october 1984 between Vicky Nelson and Jean Hepner which lasted 29 minutes(643 strokes)!!!
All in all, the match lasted 6 hours and 22mins. And the tiebreak in that match lasted 1hour 47mins
I bet this match was exciting:retard:
UPDATE: It was in the first round of that tourney and the final score was 6:4, 7:6 (13:11).