World No.1 Naomi Osaka and Simona Halep have won their opening matches at the Madrid Open, while Garbine Muguruza was upset by Croatian ace Petra Martic.

The top-ranked Osaka, the reigning US Open and Australian Open champion, hit 43 winners, including eight aces on the outdoor red clay to beat 2016 finalist Dominika Cibulkova 6-2 7-6 (8-6) on Sunday.

“For me, I’m just happy to get through it,” Osaka said.

“I feel the most nervous during the first round and to play against Cibulkova was kind of tough for me, especially on clay.”

Osaka will face Sara Sorribes in the second round after she got past fellow Spanish wild card Lara Arruabarrena.

Martic ousted two-time grand slam winner Muguruza 7-5 7-6 (7-2) a week after the Croat won her first career title in Istanbul.

Two-time Madrid champion Halep brushed off Russian qualifier Margarita Gasparyan 6-0 6-4.