Serena Williams and Simona Halep out of Wuhan Open

Serena Williams had her temperature and blood pressure checked before retiring

World number one Serena Williams retired from her Wuhan Open second-round match because of a viral illness.

The top seed, 32, was leading France's Alize Cornet 6-5 in the first set when she received treatment before withdrawing from the event in China.

"I felt dizzy and nauseous in the first set and unfortunately couldn't continue," Williams said in a statement in which she apologised to fans.

Romanian second seed Simona Halep lost 2-6 6-2 6-3 to Garbine Muguruza.

Williams, who won the US Open this month, was troubled by illness at Wimbledon in July, when she withdrew from the women's doubles.

Cornet, the world number 21, also beat American Williams at Wimbledon and in the Dubai Championships.

Tenth seed Jelena Jankovic also retired from the tournament. The Serb was trailing 4-1 when she quit against American player Coco Vandeweghe because of a back problem.

Eleventh seed Sara Errani of Italy, number 13 seed Ekaterina Makarova and 16th seed Andrea Petkovic of Germany were also knocked out.

However, Wimbledon champion and third seed Petra Kvitova of the Czech Republic crushed Italian Karin Knapp 6-3 6-0 and Canadian sixth seed Eugenie Bouchard overcame a break in the third set to beat Germany's Mona Barthel 4-6 6-3 6-4.

Seventh seed Angelique Kerber of Germany enjoyed a 4-6 6-2 6-4 win over Zarina Diyas of Kazakhstan, and eighth seed Caroline Wozniacki held off Spaniard Carla Suarez Navarro 6-1 3-6 7-6 (7-4).

Five seeded players went out on Monday, the opening day, including fifth seed Agnieszka Radwanska and ninth seed Ana Ivanovic.

Meanwhile, British number two James Ward lost 6-4 3-6 7-6 (7-5) to Germany's Philipp Petzschner in the first round of the Malaysian Open.

Wimbledon champion Petra Kvitova shakes hands with Karin Knapp of Italy after their match