US ninth seed Williams, seeking her third US Open crown, advanced to a Thursday matchup against 83rd-ranked compatriot Sloane Stephens for a spot in Saturday's final.
"I'm so fortunate to have won that match. It came down to the wire," said Williams. "I try to tell myself enjoy the competition, enjoy the battle and I think I was able to do that."
In the men's event, South Africa's Kevin Anderson reached a Grand Slam semi-final for the first time where he will face Spain's Pablo Carreno Busta, who will also be appearing in his maiden last-four at a major.
Stephens, who missed 11 months with a left foot injury before returning at Wimbledon, matched her best Grand Slam showing by outlasting Latvian 16th seed Anastasija Sevastova 6-3, 3-6, 7-6 (7/4).
"This is just incredible," said Stephens. "When I started my comeback at Wimbledon, I could never have dreamed of coming back and having these results. It's indescribable."
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Stephens has won 13 of her past 15 matches and matched her 2013 Australian Open semi-final run for best Slam effort.
Williams, who improved to 2-4 all-time against the Czech 13th seed, will return to the world top five for the first time since January 2011, a span that saw her diagnosed with the energy-sapping disease Sjogren's Syndrome.
Williams, the oldest Slam semi-finalist since Martina Navratilova at Wimbledon in 1994, won the 2000 and 2001 US crowns.
She was runner-up this year at Wimbledon and the Australian Open and could reach three Slam finals in a year for the first time since 2002.
Kvitova battled back from a five-month absence after a knife-wielding home intruder injured her left hand last December.
"I'm glad I'm still able to compete on the high level against the top players which I hope will give me some confidence to continue to play well, to know that it's still there somewhere," said Kvitova.
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