World number one Naomi Osaka was tested by Maria Sakkari in her first round match at the Nature Valley Classic in Birmingham on Tuesday.
But she eventually went through to the last-16 of the grasscourt tournament with a hard-fought 6-1 4-6 6-3 win.
In her first match since her third round exit at Roland Garros earlier this month, the 21-year-old Osaka struggled.
She battled with her first serve against the world number 33 and took two hours to dispatch her Greek opponent.
However the Japanese, who lost to Sakkari prior to her U.S. Open title run last year, did manage to fire seven aces.
She won five breakpoints to clinch victory on a surface she had previously admitted was not her strongest.
Elsewhere, former French Open champion Jelena Ostapenko stormed into the next round with a 6-0 6-2 demolition of Iga Swiatek.
Petra Martic saw off the challenge of Ekaterina Alexandrova with a 6-3 7-5 win.
Venus Williams’ match against Aliaksandra Sasnovich was delayed due to rain.
Second seed and French Open champion Ashleigh Barty was set to take on Donna Vekic later on Tuesday.