Osaka loses in Berlin as Zheng breaks aces record

"I was trying to improve the consistency of the serve because I know it's so important, especially on grass courts. One break and you win the set, one break and you lose the set. That's precisely what happened today," Zheng said.

Another former world No.1 who has become a mother, Angelique Kerber, also lost.

Playing her first grass court match since a return from maternity leave the former Wimbledon champion lost 7-6 (7-4) 2-6 6-4 to Czech teenager Linda Noskova.

Last year's finalist Donna Vekic ousted Jule Niemeier 1-6 6-1 6-3 which means Germany have no player left at the home event.