Ninth seed Williams spent only 28 minutes on court against Flavia Pennetta before the Italian was forced to retire with a right wrist injury, with Williams leading 4-0 in the first set.
Meanwhile eighth seed Li came through against Slovakia's Dominika Cibulkova, coasting the first set then holding her nerve in a tie-break to triumph 6-1 7-6 (7/4) in one and a half hours.
Williams told wta tour.com: "Li Na is the defending French Open champion. She's great to test yourself against because she's a good all-round player and she's so fit. But I've played on ice and water - I can play on any surface."
Williams' sister Venus was denied the chance to also move into the semi-finals as she fell 6-4 6-3 to second seed Maria Sharapova. The Russian served well throughout and saved two out of three break points en route to victory.
Sharapova will face Germany's world number 24 Angelique Kerber, who stunned fourth seed and Wimbledon champion Petra Kvitova after coming through a see-saw, two-and-a-quarter-hour contest to win 7-6 (7/1) 1-6 6-1.