Japan's world No. 1 Naomi Osaka advanced to the second round of the Madrid Open on Sunday by beating Dominika Cibulkova in straight sets.

Osaka, playing in her first match since withdrawing from April's Porsche Tennis Grand Prix semifinal due to abdominal pain, showed she has returned to top fitness but had to fight hard in the second-set tiebreaker to dispatch the world No. 33 Slovak 6-2, 7-6(6).

"I feel good. I don't really have that much pain in my abs so that's always a good sign," she said. "I'm just really happy to be out here and playing against Cibulkova."

"She's someone I actually watched growing up, so super fun."

(Naomi Osaka (L) and Dominika Cibulkova)

The top-seeded Osaka, who won her second Grand Slam at this year's Australian Open, fired 43 winners against Cibulkova's 23, while making twice as many unforced errors than her opponent in the match that lasted 1 hour, 55 minutes.

Osaka claimed her first main-draw win at this clay-court tournament in Spain, having lost in the opening round last year after falling in the qualifiers at the 2016 meet.

In the second round, Osaka will face the winner of an all-Spanish match between wild cards Lara Arruabarrena and Sara Sorribes Tormo.

In the pair's third meeting, Cibulkova, the former world No. 4, broke serve first to take a 2-1 lead in the first set, but Osaka returned the favor immediately before claiming four more games in a row to close the set.

The 21-year-old Osaka had to come from behind to level at 4-4 in the second set, but managed to maintain her perfect 3-0 record against Cibulkova after the second-set tiebreaker.