Qualifier Olga Govortsova of Belarus was leading their second-round contest 6-0 and 2-1 when Poland's Radwanska, the Wimbledon runner-up, called it a day. Radwanska, who enjoyed a first-round bye, had received treatment on the shoulder after the first set.
Govortsova advanced to a quarter-final clash with Russian seventh seed Maria Kirilenko, who advanced on a walkover when Germany's Mona Barthel withdrew before their scheduled match with a stomach ailment.
Earlier, French Open finalist Sara Errani and former Wimbledon runner-up Marion Bartoli booked a quarter-final meeting.
Italy's Errani, the fourth seed, defeated Spain's Carla Suarez Navarro 6-4, 6-3, while fifth-seeded Bartoli of France survived a scare to beat American Sloane Stephens 6-1, 0-6, 6-3.
Bartoli trailed 0-3 in the third but won the next six games to advance in the US Open tune-up event.
Despite the mountain facing her in the third, Bartoli said she never lost heart.
"I started to hit the ball harder -- (to) be mentally very tough," she said, although she admitted she had expected things to be much easier after she breezed through the opening frame in 21 minutes.
"I really thought after the first set, I was just on a run," Bartoli said.
Stephens, however, stormed back, winning the first game of the second set and breaking Bartoli in the next as the French player double-faulted twice.
Bartoli's serve was a weak spot throughout the second set, in which she double-faulted six times.
She surrendered a break in the second game of the third set with another double fault, but finally held serve in the fourth game to gain the toe-hold she needed.
"Of course my experience helped me, but also I felt like I was able to increase my level of intensity and not make a lot of mistakes," Bartoli said.
First-round action concluded Tuesday as China's Zheng Jie defeated Bulgarian Tsvetana Pironkova 7-5, 6-2 to set up a second-round match with eighth-seeded Czech Lucie Safarova.