World number one Williams, a 16-time Grand Slam winner seeking her fifth US Open crown and her ninth title of the year, dispatched 18th seed Carla Suarez Navarro 6-0, 6-0, in 52 minutes on the Spaniard’s 25th birthday.
“I’m so happy to be in the semi-finals. It really feels great,” Williams said. Li, the 2011 French Open champion, overcame nervous feelings for a 6-4, 6-7 (5/7), 6-2 victory over Russian 24th seed Ekaterina Makarova.
Reigning French Open champion Williams is 8-1 all-time against Li but was tested 7-5, 7-5 in their most recent matchup last month in a US Open run-up semi-final at Cincinnati.
“She is playing some of the best tennis of her career,” Williams said. “It’s going to be a tough match,” she added.
Li said: “I’m not so happy. Today was the first time I was feeling so nervous. I was happy because at least I was fighting a lot on the court. “Didn’t play the best of tennis but I still fought am still in the tournament.”