"以发球赢一分" can be translated into English as "to win a point with a serve" or more naturally as "to secure a point through a serve".
Here are two example sentences to illustrate the usage:
1、 She managed to win a point with a powerful serve.
(她以一个有力的发球赢了一分。)
2、 He secured a point through a well-placed serve that his opponent couldn't return.
(他通过一个落点极佳的发球,让对手无法回击,从而赢了一分。)