A smiling Naomi Osaka referred to an apparent injury scare at the Australian Open as "not that big of a deal" after moving into round three with a straight-sets win over Tamara Zidansek.

Fourth seed Osaka, who claimed her maiden grand slam title with victory in the US Open last September, dropped just six games for the second match in succession on Thursday.

However, the Japanese did call for the trainer in the second set against Zidansek before completing a 6-2 6-4 victory.

Asked to explain what the issue was at a post-match news conference, Osaka said: "I was able to finish and win the match, so it's not that big of a deal."

"It's something that I have to keep an eye on. I'm not telling you, though."

Osaka has been hampered by a back injury recently and was pressed on whether that was the problem.

"You'll never guess. You'll never find out," she replied with a grin.

The 21-year-old faces Hsieh Su-wei, the 28th seed, in round three.