World No.1 Naomi Osaka was tested by Maria Sakkari in her first-round match but eventually went through to the last-16 with a hard-fought 6-1 4-6 6-3 win.
Second-ranked Ashleigh Barty could overtake Osaka at the top of the rankings if she wins the title and Osaka falls before the semifinals. If Osaka makes the final, she will secure the No.1 ranking and be the provisional top seed for Wimbledon.
In her first match since her third-round exit at Roland Garros earlier this month, 21-year-old Osaka struggled with her first serve against the world No.33 and took two hours to dispatch her Greek opponent.
However, the Japanese, who lost to Sakkari prior to her US Open title run last year, did manage to fire seven aces and won five break points to clinch victory on a surface she had previously admitted was not her strongest.
Elsewhere, former French Open champion Jelena Ostapenko stormed into the last 16 with a 6-0 6-2 demolition of Iga Swiatek while Petra Martic saw off the challenge of Ekaterina Alexandrova with a 6-3 7-5 win.
Ostapenko will next face British No.1 Johanna Konta for a place in the quarterfinals.
Barty’s encounter with Donna Vekic was pushed back a day after play was ended prematurely at the WTA Birmingham Classic because of heavy rain.
Newly-crowned French Open champion Barty was due to open her grass-court singles campaign against Nottingham Open finalist Vekic but tournament organisers decided on an early end to proceedings with no let-up in the drizzle.
Her Centre Court match with the Croatian will now open proceedings on Wednesday at 11.30am local time.
That match will be followed by Venus Williams, who faces Aliaksandra Sasnovich.
Here's the revised order of play for tomorrow's #NatureValleyClassic ????
— LTA (@the_LTA) June 18, 2019
Other winners on Tuesday included No.8 seed Julia Goerges, Yulia Putintseva and qualifier Kristyna Pliskova, who set up a second-round meeting with her twin sister – No.3 seed Karolina Pliskova.
“I just checked all the scores we played. I’m not sure about how it will be because it’s very different than to face someone else,” Kristyna told wtatennis.com.
“So many people were talking about it already yesterday, and I was like, ‘Come on, I still have one more match to go, so don’t skip.’ But so far I’m feeling good, so they were expecting me to win today as well.
“It’s very strange to play her. It’s family, so you don’t want (to play) her. But still I want to win as well.”
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