Activision Tennis is played for one set of games, in which the first player to win six games won the competition, unlike real life tennis, where competitors have to win, at least, two out of three six game sets.
One of the players on the screen appears in pink; the other one in blue. Most of tennis' rules applied to this game, except that there are no out of bounds shots. Shots that hit the lines were always considered inbound shots. Due to technical limitations (low resolution), the tennis ball appears as a block instead of a sphere.
Source: Wikipedia, "Activision Tennis", available under the CC-BY-SA License.