Radwanska out of US Open
Agnieszka Radwanska of Poland serves against Shuai Peng of China during the 2014 US Open on August 27, 2014 in the Queens borough of New York City - by Julian Finney
Radwanska, the champion in Montreal in the run-up to the tournament, had also lost to the Chinese player at the same stage in New York in 2010.
Her defeat continued her miserable record at the US Open where she has never got beyond the fourth round.
Peng, the world number 39 who made the fourth round in 2011, goes on to face either Roberta Vinci of Italy or Irina-Camelia Begu for a place in the last 16.