Williams, the defending champion seeking her fifth U.S. Open title, overwhelmed No. 18 seed Carla Suarez Navarro of Spain 6-0, 6-0 in a 52-minute, fourth-round mis-match.
The American star hardly broke a sweat in winning 88 percent of her first serve points.
She now goes on to face China's Li, who earned her first spot in the U.S. Open semis with a 6-4, 6-7 (5-7), 6-2 decision over No. 24-seeded Ekaterina Makarova in a match that took nearly 2 1/2 hours.
Li didn't face a break point in the third set and finished the match with her seventh break of the day to advance to a Grand Slam semifinal for the fifth time. She's 3-1 in her previous major tournament semifinals but just 1-8 in her career against Williams.
Meanwhile, Belarusian Victoria Azarenka, who lost to Williams in the 2012 U.S. Open final, needed guile to rally past 13th-seeded Ana Ivanovic 4-6, 6-3, 6-4 in a fourth-round match that had been scheduled for Monday but was pushed back due to a poor weather forecast.
Neither player had a clear advantage for the bulk of the match. Ivanovic fought off a pair of match points and broke Azarenka's serve to get within 4-5 in the third set but Azarenka was able to record her ninth break of the day -- and four of the third set -- to close out the match.
Azarenka now goes on to meet Slovakia's Daniela Hantuchova, who squeaked past American fellow non-seed Alison Riske in three sets Monday. The pair have split four career meetings, but Azarenka has taken the last two.