Naomi Osaka reaches last 16 in New York
Fourth seed safely through in the US Open after second set wobble against Kostyuk
Naomi Osaka returns a shot during her match against Marta Kostyuk. Photograph: Al Bello/Getty Images
The Japanese fourth seed looked out of sorts in the second set tiebreaker but regrouped to secure the victory in the deciding set after two hours and 33 minutes against an 18-year-old who was making her US Open main draw debut.
Osaka, whose left leg was heavily taped after suffering a hamstring injury that forced her to withdraw from the final of a tune-up event last week, wasted little time in the decider as she went up a double break before serving out the match at love.
Angelique Kerber is also through to the last 16 following a 6-3, 6-4 victory over 20-year-old American Ann Li. Kerber, a three-times Grand Slam champion who won the title at Flushing Meadows in 2016, converted four break points while Li - who is ranked 128 in the world - was let down by 28 unforced errors.
Li did however give a good account of herself, taking risks with some powerful forehand winners that caught the 17th seed Kerber off guard, but the 32-year-old German dictated play on the crucial points and was broken only once in the contest.
And Croatian eighth seed Petra Martic survived another sloppy display to see off Grand Slam newcomer Varvara Gracheva 6-3 6-3. Martic, who struggled through an error-riddled match against Kateryna Bondarenko in the second round, was again often her own worst enemy, allowing her 20-year-old Russian opponent to stay in the contest.
The match got off to a ragged start with three straight breaks before Martic managed to hold serve and grab a 3-1 lead. While Gracheva continued to pile up the errors, Martic began to settle into the match, holding her serve and breaking the Russian a fourth time to take the opening set.
However, it was more of the same in the second with the pair trading breaks before Martic took control, moving into a 4-1 lead and holding on for an uneven win.
After victories over two players outside the top 100, Martic will have to clean up her play with Grand Slam champions Osaka, Kerber and Petra Kvitova lurking on her side of the draw.