The first few games will go to Bunta easily - he's not holding anything back, and playing tennis like no one has ever seen before. Eventually, Seiichi will "wake up," so to speak, and give his full effort in return, and start to win some games. Regardless of the score or how many sets they've played, they will not stop until one of them collapses.
It will end when Seiichi has to throw in the towel, his stamina can't keep up with Bunta's, and at that point, Bunta will have been losing for a while (though not without a damn good fight). He'll probably just crumble, too, once he's satisfied that Seiichi has given in. They'll both be so totally unaware of their surroundings, that they would not have noticed the crowd (or familiar faces in the crowd) or the passing of time.
At that point, they'll gimp off to the showers, and then Bunta's going home.