WUHAN (China) • Serena Williams' withdrawal from the Wuhan Open was "unfortunate", the tennis tournament's director said yesterday, after the American star ruled herself out for the second year in a row.

The 22-time Grand Slam champion pulled out of the Chinese event - and next week's China Open in Beijing - with a shoulder injury, robbing it of its best known player.

"It's unfortunate but we understand. It's the nature of the sport," Fabrice Chouquet said. "We hope to see her here in 2017."

Williams has made it to the hot and humid Wuhan Open only once, in 2014, when she retired from her first match against Alize Cornet with illness. Last year, she was also injured and skipped the entire Asian swing, a busy period which culminates in the season-ending WTA Finals in Singapore.

The Wuhan Open, held just after the year's final Grand Slam tournament, the US Open, has become bedevilled with injury problems, witnessing a series of retirements.

Six players retired mid-match in the first two days of this year's tournament, while another five suffered the same fate last year - including Spaniard Garbine Muguruza in the final.

"We are towards the end of the season and we know the players have played a lot and we know that some of them are very tired," said Chouquet. "A player never retires by pleasure. They retire because they have to."

Women's Tennis Association chief Steve Simon has vowed "fundamental changes" to the sport's gruelling schedule, which has added eight tournaments in China since 2008.

"The circuitry structure is on the agenda," Chouquet said, adding that there could be changes to the schedule by the 2018 season.

"It's always a very fine line (between) giving playing opportunities to players and overloading a calendar... (and for) these top tournaments to receive the player field they deserve for the investment they put (in)."

Yesterday, Angelique Kerber survived a scare in her first match as world No. 1 before beating Kristina Mladenovic in three sets.

The US Open and Australian Open champion, who had a bye in the first round, made a convincing start before missing two set points and allowing Mladenovic to take the first set in a tie break.

But the German then swept through, winning 6-7 (4-7), 6-1, 6-4, to set up a third-round meeting with two-time Wimbledon champion Petra Kvitova today.

"I was trying to find my rhythm because I was not playing my best in the first set," top seed Kerber said after the match.

World No. 4 Agnieszka Radwanska romped into the third round with a 6-4, 6-1 win against Russia's Ekaterina Makarova, who bagged Olympic women's doubles gold last month in Rio.

But last year's US Open finalist Roberta Vinci became the tournament's first seeded casualty when the 15th-ranked Italian was ousted by Kazakhstan's Yaroslava Shvedova 7-5, 6-2.

AGENCE FRANCE-PRESSE