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.
Beyonce flaunts bikini body, Blue Ivy in vacation pics
Couple mistakenly served bag of cash at McDonald's drive-thru