2.6.4 From the start of service until it is struck, the ball shall be above the level of the

playing surface and behind the server's end line, and it shall not be hidden from

the receiver by the server or his or her doubles partner or by anything they wear

or carry.

只要球一直能被对手看见，就合规。伊藤的发球很刁钻，我觉得主要是她击球快，让对方看不清是转还是不转，是下旋还是上旋侧旋。