World number one Naomi Osaka came through a tricky test against Greece's Maria Sakkari to reach round two at the Nature Valley Classic in Birmingham.

Osaka, who before the tournament said she was "not really that comfortable" on grass, had looked in early control before battling to a 6-1 4-6 6-4 win.

The Japanese 21-year-old will face Yulia Putintseva next after the Kazakh beat British wildcard Harriet Dart.

"I was kind of screaming on the inside during that entire match," Osaka said.

"I got lucky a little bit I guess. I knew she was going to be a very difficult opponent."

US and Australian Open champion Osaka found herself 0-40 on her serve at 2-2 in the final set but, after saving six break points in a near 10-minute game, finally held.

It proved pivotal as she went on to break the Greek world number 33 in the following game before holding to love and then going on to seal victory on her first match point as the clock ticked past two hours.

Britain's Dart was well beaten by Putintseva, who is ranked 114 places above the British world number 157, losing 6-1 6-4.

The 22-year-old Briton had a glimmer of hope when she broke early in the second set but could not match Putintseva's consistency.

Eighth seed Julia Gorges is also through to the second round after coming from behind to beat Ukrainian Dayana Yastremska 3-6 6-4 6-3.

British number one Johanna Konta, who beat Anett Kontaveit on Monday, found out her second-round opponent will be former French Open champion Jelena Ostapenko after the Latvian took just 52 minutes to beat Polish qualifier Iga Swiatek 6-0 6-2.

Seven-time Grand Slam champion Venus Williams is in action later on Tuesday against Belarusian Aliaksandra Sasnovich, although play was suspended at the Edgbaston Priory Club soon after Osaka's win.

The weather has also disrupted Tuesday's schedule at the men's grass-court event at Queen's, where no tennis had been played by 16:15 BST.