A player serving in tennis has two chances to get a serve into play. If the first serve is out, the player serves again. If the second serve is also out, the player loses the point. Here are the probabilities based on a long running championship series:
P(1st serve is in) = $firstservein
P(win point | 1st serve is in) = $firstservewin.
P(2nd serve in | 1st serve out) = $secondservein.
P(win point | 1 serve out and 2nd serve in) = $winpoint.
Consider the (partially) completed tree diagram below: