That's highly likely but no one can tell how much pain someone else is going through or what's going on inside someone else's head. Everyone has a different pain threshold. Sometimes we try to put a brave face on things & work through the pain but can't quite make it. It might have been an excuse because he knew he was being thrashed & Federer was the better player but didn't want to accept that but no one will ever know for sure.If Chung was leading in the match, he'd have handled the pain. He quit because he realised he had no chance of winning and Federer was running him ragged.