The 18-year-old beat the ninth seed and former French Open champion in three sets to become the first British woman in 16 years to reach round four of a Grand Slam.
"It was just an extraordinary performance," Croft told Sky Sports News. "To be able to back up what she did against Kim Clijsters a couple of days ago.
"I thought the response of losing that tie-break and the manner was so impressive."
Robson's reward for knocking out Li is a last-16 encounter with defending champion Samantha Stosur.