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.

只要球一直能被對手看見，就合規。伊藤的發球很刁鑽，我覺得主要是她擊球快，讓對方看不清是轉還是不轉，是下旋還是上旋側旋。