Czech sixth seed Petra Kvitova is out of the US Open after losing 6-4 6-4 to world number 88 Andrea Petkovic in the second round.

The two-time Wimbledon champion, who has been struggling with a forearm injury, was broken in the 10th game as the German, 31, took the opening set.

She was a break up in the second set before Petkovic broke twice more en route to sealing victory.

Japanese defending champion Naomi Osaka defeated Pole Magda Linette 6-2 6-4.

Linette, the world number 53, was 3-0 up in the second set before the world number one fought back.

Osaka could face promising 15-year-old Coco Gauff next, with the American playing Hungary's Timea Babos in her second-round match from 00:00 BST on Louis Armstrong Stadium.

Petkovic will now play Belgian 25th seed Elise Mertens.

Elsewhere, Ukrainian 32nd seed Dayana Yastremska beat Swede Rebecca Peterson 6-4 6-1 to set up a third-round meeting with compatriot and fifth seed Elina Svitolina.

Tunisia's Ons Jabeur, who knocked out 27th seed Caroline Garcia in round one, fought back from a set down to beat Belarus' Aliaksandra Sasnovich 3-6 6-4 6-2.

It is the first time Jabeur has reached the third round of the US Open and she will play 2016 finalist and third seed Karolina Pliskova.

Chinese Taipei's Hsieh Su-Wei, the 29th seed lost in a third set tie-break as Karolina Muchova of the Czech Republic won 6-1 4-6 7-6 (7-2) and will play seven-time champion Serena Williams.

Greece's Maria Sakkari lost the first set against China's Shuai Peng before triumphing 6-7 (5-7) 6-4 6-2. The 30th seed will now take on world number two Ashleigh Barty, the French Open champion.