ROME, Italy - With two matches potentially on her plate after rain washed out Wednesday's play in Rome, World No.1 Naomi Osaka was efficient in a delayed start to her tournament at the Internazionali BNL d'Italia on Thursday.

For the second straight week, Osaka beat Dominika Cibulkova in straight sets, 6-3, 6-3, to book her spot in the round of 16 against either Mihaela Buzarnescu later today.

The Romanian advanced when No.16 seed Julia Goerges retired in the third set of their match due to a right thigh injury with the score reading 6-4, 3-6, 4-4.

Behind a whopping 44 winners and 12 aces, Osaka extended her perfect head-to-head against the Australian Open finalist to 4-0, having never lost a set.

"I feel pretty good today. I was expecting a really hard match. That's what I was given. Given the circumstances, I feel pretty great," Osaka said.

"I think for me today, especially my serve was really good. I just tried to be really calm."

Osaka became the first winner on a stacked Thursday slate in an hour and 43 minutes, coming from an early break down in the first set after losing serve for the only time in the match at 1-1.

The World No.1 was forced to battle hard to keep her lead, however, as she saved break points at 2-2 and 3-3 before finally converting her third chance on the Slovak's serve in the eighth game to break for the second time.

From there, Osaka kept the former World No.4 at arms' length, breaking serve to start the second set and staying ahead for the duration.

She saved all four break points she faced in the second set, including three in a key eighth game which would've seen Cibulkova pull even and get back on serve.

The Slovak, a former quarterfinalist in Rome seven years ago, did not play poorly in the match overall, finishing with a positive winner-to-unforced error ratio to 17-14.

Nonetheless, she managed to win just one of the eight break points she created in the match, and allowed Osaka a whopping 15 break point chances on her serve overall.

Osaka boasted a winning head-to-head against either of her round of 16 opponents, and she Buzarnescu in their only career meeting on grass last June in Birmingham.