French Open champion Jelena Ostapenko is out of the US Open, losing in the third round 6-3 6-2 to Russian Daria Kasaktina after hitting 38 unforced errors on Saturday.

The Latvian 12th seed, who broke into the limelight in June when she blasted her way to the Roland Garros title, got off to a fast start but the unseeded Kasaktina mixed her game with sliced backhands and chopped forehands to slow her opponent down.

The plan worked as after winning the first three games, Ostapenko grew frustrated and bowed out when she netted a backhand on the second match point at the end of a lung-busting rally.

Ostapenko complained of nausea and illness during the match.

“In the beginning I was a little bit nervous but I tried to stay focused and play my game and it paid off,” said Kasaktina, who faces Estonian qualifier Kaia Kanepi for a place in the quarter-finals.

Karolina Pliskova showed the steel of a world No.1 as she fought off a match point to claim a riveting 3-6 7-5 6-4 comeback win over Zhang Shuai.

Pliskova, runner-up at Flushing Meadows last year, would have lost her world number one ranking had she been beaten on Arthur Ashe Stadium but instead moved another step closer to a first career grand slam title.

China’s Zhang was the more aggressive of from the start, frequently rushing to net and stepping into the court to attack Pliskova’s second serve.

The strategy paid off as she broke Pliskova in her first two service games to take a 3-1 lead and captured the first set with an emphatic overhead smash.

She continued her fiery play into the second set but was unable to convert a break point and mental lapses started to pile up.

Zhang ended the contest with 46 unforced errors to Pliskova’s 38.

She was also undone by the tall, big-serving Czech’s nine aces to just one double fault while Zhang hit just two aces and made five doubles.