The seven-time champion, who missed the tournament a year ago, was outplayed by Italian Camila Giorgi initially but fought back impressively to win 3-6 6-3 6-4 on Centre Court.
Williams took control of the third and final set in the the third game, claiming the third break of the match, and with the remaining games all going to serve, served out for a 3-6, 6-3 6-4 win.
Li Na said she was stunned by Serena Williams' impressive comeback from giving birth and said her old rival has a golden chance to reclaim her Wimbledon crown.
But from then on the seven-time champion lost only nine more points on serve and finished with three love holds to make an 11th semifinal at the All England Club.
Serena Williams figures this statistic needs an asterisk: For the first time since Wimbledon started seeding players in the 1920s, none of the top 10 women reached the quarterfinals. "I play a great level today", she said.
Against the other two players in the semifinals, Williams has never played No. 12 seed Jelena Ostapenko and has won six of her eight matches against Angelique Kerber, the No. 11 seed.
Williams stepped up to the line at 5-4 and brought up match point with an ace before completing victory, her 91st here, when Giorgi pushed a forehand into the net.
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After sweeping aside Russian Evgeniya Rodina 6-2 6-2 on Centre Court, Williams, 36, had an ominous warning for her rivals.
She will face German world number 10 Kerber on Saturday in a repeat of the 2016 showpiece won by Williams. It's also a first for any Grand Slam tournament in the 50-year professional era. "It's a great opportunity for me to meet her at that stage".
"I faced a thousand and three seeds in my life, so I'm okay with that", Williams responded when asked if she would have preferred stiffer tests.
In June, she was a match away from the French Open quarterfinals before withdrawing from the tournament an hour before her anticipated match against Maria Sharapova.
Ostapenko roared into her first Wimbledon semi-final with a display of power-hitting against Dominika Cibulkova.
After losing the first set in the quarterfinals Tuesday, Serena Williams looked dejected and confused. The match between Germany's Julia Goerges and Netherland's Kiki Bertens went the full hull.
It was the first frame she dropped during her 2018 campaign and ended an astounding 20-set win streak at the All England Club, dating back to 2016. "For me, there's so much farther I want to go to get back to where I was, and hopefully go beyond that". The last time two German women reached any Grand Slam semifinals was at the 1993 French Open, featuring Steffi Graf and Anke Huber. "Well, it's still one more match to go for both of us". It appeared nearly to be a reminder to Giorgi, who said on Monday that she doesn't watch women's tennis.