Heavy rain sent the players running for cover at 6-6 in the decider, shortly after the Russian had fended off a match point with a winning forehand.
When they finally returned on a damp night in the Italian capital, Sharapova won the first three points of the tiebreak. Although China's Li battled back to 4-4 and 5-5, the second seed grabbed the last two points for victory.
"This is a great win for me going into Roland Garros," Sharapova told reporters.
Image: Russia's Maria Sharapova celebrates with the trophy after defeating China's Na Li at the Italian Open tennis finals, in Rome, Sunday, May 20, 2012. Sharapova won 4-6, 6-4, 7-6 (5).