Defending Australian Open champion Naomi Osaka has made a winning start to her summer with a hard-fought opening round victory at the Brisbane International.

Osaka defeated Greece’s Maria Sakkari 6-2 6-7 (4-7) 6-3 on Tuesday to set up a second-round encounter with American Sofia Kenin.

The two-time grand slam champion and former world No.1 took over two hours to overcome the in-form Sakkari, hitting 16 aces on her way to the victory.

In other action on Tuesday, American eighth seed Madison Keys eased past Czech qualifier Maria Bouzkova 6-2 6-3.

Keys will now face local favourite Samantha Stosur in the second round.

“Sam’s always tough,” Keys said.

“I think she’s beaten me every time we have played.

“It’s also hard to play someone who is going to be playing at home and have the crowd and all that.”

Russian qualifier Liudmila Samsonova produced the upset of the tournament so far, defeating American Sloane Stephens 6-4 2-6 6-3.

Samsonova will face the winner of Tuesday evening’s clash between two-time Wimbledon champion Petra Kvitova and Russia’s Anastasia Pavlyuchenkova in the second round.

Wildcard entry Maria Sharapova is set to play her opening round fixture later on Tuesday against American qualifier Jennifer Brady.