Sloane Stephens suffered an early exit at the Wuhan Open as Naomi Osaka was forced to withdraw from the tournament.

American Stephens, the ninth seed, fell at the first hurdle despite winning the opening set against Anett Kontaveit in the final match on Sunday.

The 2017 US Open champion led 4-2 at one stage in the second only to be pegged back by her Estonian opponent, who broke again in the 12th game to force a decider.

Both players lost their serve midway through the third set before Kontaveit - who won just two games when the pair met at the French Open earlier this year - broke to love to seal a 4-6 7-5 6-4 triumph.

Osaka, meanwhile, will not be present in China due to illness, the US Open champion making the decision not to travel after losing in the final of the Tokyo Open.

MIXED FORTUNES FOR SEEDS

Stephens was not the only notable name to be on the wrong end of an upset; Elise Mertens, seeded 15th, lost in straight sets against Aliaksandra Sasnovich.

However, 13th seed Daria Kasatkina overcame Lesia Tsurenko in straight sets to progress, a 6-4 7-5 result avenging a loss against the Ukrainian at Birmingham earlier in the year.

Julia Goerges, the 11th seed, was taken to three sets by Marketa Vondrousova, eventually winning 6-3 3-6 7-6 (7-4) after two hours and 40 minutes on court.

BIG NAMES RULED OUT

Tournament organizers suffered a blow with Osaka's announcement, lucky loser Polona Hercog taking her place in the main draw.

The world number seven had lost in straight sets to Karolina Pliskova in Tokyo in her first event since lifting the trophy at Flushing Meadows.

"I'm very sorry to have to withdraw from Wuhan. I look forward to playing in Wuhan next year," Osaka said in a statement.

Former world number one Victoria Azarenka had already pulled out because of a viral illness, while a respiratory problem has put paid to Magdalena Rybarikova's hopes of competing.

Still, a strong field includes the top five in the world, led by the top-ranked Simona Halep, who has received a bye through to the second round.