The ninth seed and former French Open champion was downed in three sets by the 18-year-old Brit inside the Louis Armstrong Stadium.
"She has a huge first serve, but it's not like I didn't have a chance" Li told reporters. "I was making a lot of mistakes in the whole match, so I gave her a lot of free points. That made her like, 'Oh, I've got more confidence. Oh, I can beat her.'"
Robson's reward for beating Li is a last-16 clash with defending champion Samantha Stosur.