(Reuters) - American Alison Riske fought back from a set down to beat Germany's Angelique Kerber 5-7 6-4 7-6(6) in the first round of the inaugural Zhengzhou Open on Tuesday, extending the former world number one's poor run of form.
Riske held her nerve to save a match point in the third-set tiebreak before going on to inflict a fourth consecutive first-round defeat on Kerber, who was beaten by Kristina Mladenovic in her first match at the U.S. Open.
The German, a three-times Grand Slam champion, was beaten in the second round at Wimbledon and suffered first round defeats in Toronto and Cincinnati in the run-up to the year's final Grand Slam at Flushing Meadows.
Mladenovic made short work of Chinese wildcard Duan Yingying earlier in the day, winning 6-2 6-3 to set up a second round meeting with fellow Frenchwoman and her former doubles partner Caroline Garcia.
"It was very tricky conditions. It's super humid and it's never easy to start a tournament," said Mladenovic, who won 79% of her first serve points during the match.
"Every first round is difficult and Yingying, I've played her in the past and she's a very powerful player with a big serve. I think the key today was the consistency on my serve, and I managed to read hers well, which is her biggest weapon."
(Reporting by Shrivathsa Sridhar and Rohith Nair in Bengaluru; Editing by Christian Radnedge)