Eighth seed Li beat Iveta Benesova with ease, 6-1, 7-5 but Czech Petra Cetkovska caught Poland's Radwanska with a 6-4, 4-6, 6-1 loss.
The winner attacked and went for her shots on a windy day on the clay at the Foro Italico, producing 44 winners and 38 unforced errors.
Of Radwanska's eight defeats in 2012, six have come from world number one Victoria Azarenka, with their most recent meeting at the Madrid semi-finals last weekend.
Li will face the winner from New Zealand's Marina Erakovic and South African Chanelle Scheepers.
In other second-round results, German 12th seed Angelique Kerber defeated American Vania King 6-3, 6-2, while Sorona Cirstea of Romania beat Swede Sofia Arvidsson 6-3, 6-1 and Italy's Flavia Pennetta hammered Sloane Stephens of the United States 6-2, 6-3.
Li Na advances; Radwanska knocked out of Rome Masters
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