Radwanska had mentioned the injury after losing in straight sets to Li Na in Cincinnati the previous week, but told reporters afterwards it was down to having been active on the tour in 2012.
"It's just inflammation," she said. "I really put on the shoulder a lot of tennis the last couple of weeks and my shoulder is saying 'no' sometimes when I go on the court."
"The Grand Slam is the most important tournament, obviously. I was just scared to play more."
Maria Kirilenko will take on Olga Govortsova in the quarter-finals, after the seventh seed also benefited from a walkover with Mona Barthel unable to compete.
The German could not play their second-round match due to a stomach illness.
Fourth seed Sara Errani of Italy was the model of efficiency as she secured her progression through to the quarter-finals of WTA New Haven with a 6-4 6-3 win over Carla Suarez Navarro of Spain.
Errani, the world number 10, was forced to work hard to clinch the crucial break of serve in the opening set as she took it 6-4, with Suarez Navarro battling from the back of the court.
The Italian found things more comfortable in the second set and she took it 6-3 to secure her place in the last eight at Yale.
Errani will now take on fifth seed Marion Bartoli after the French player recovered from a dire second set showing to overcome Sloane Stephens of the US 6-1 0-6 6-3.
Earlier in the day, Zheng Jie of China beat Bulgaria's Tsvetana Pironkova 7-5 6-2 to complete the first round play.
- Sara Errani
- Maria Kirilenko
- Carla Suarez Navarro