With Williams serving to take the first set, Strycova managed two break points before the seven-time Wimbledon champion blasted an ace pact Strycova to take Set 1 in just 27 minutes.
Williams will face unseeded late-bloomer Barbora Strycova on Thursday for a place in the Wimbledon final, having begun the tournament short on match practice and with question marks hanging over her prospects.
"Barbora will force Serena to try many different things and move away from a Plan A".
Williams doesn't really have anything to lose, either, but she has plenty to gain. I'm motivated to win. I'm glad I was able to come through. She was honestly playing unbelievable, she beat so many great players.
"I have really nothing to lose". "It's a long, arduous road".
Strycova, meanwhile, was a surprise victor in her quarterfinal against Konta. The 33-year-old slumped to a 4-1 deficit in the first set, but rallied back to take the set and then cruised to a 7-6 (7-5), 6-1 win.
"I can say that I have a better game in these days", said Halep, when asked about her development as a player since reaching the Wimbledon semifinals in 2014. "It is one of the best moments of my life", said Halep.
"I'm not getting over-pumped, but at the same time not getting under-wound". "I don't think you need to pick on me in a harsh way".
The pair have met three times before, with Williams winning every time including in the first round at Wimbledon in 2012. As the seventh and eighths seed respectively, the two are highest ranked players left in the women's tournament.
Williams was prowling like a hungry tiger and piled on the pressure in Strycova's next service game. Six of the 10 matches have been decided in straight sets.
Svitolina blinked first, with Halep finally getting the break of serve at the third attempt, before the Ukrainian immediately broke back to love.
Saturday's final will be Williams' 11th appearance in Wimbledon singles championship, the second-most all-time behind only Martina Navratilova's 12.
Why wouldn't she? She owns a 9-1 career record against Halep, including a victory in the Australian Open's fourth round in January.
"I fought hard to win this match and I was strong mentally and physically and had the right tactics".
Svitolina, meanwhile, reached her first ever Grand Slam semifinal after beating Czech Karolina Muchova.
Williams, whose two sets win took her to 200 career sets won at Wimbledon joint second with Chris Evert in the all-time women's list behind Martina Navratilova (249), says that she is in a better frame of mind than she was when she ended up losing in both the Wimbledon and US Open finals a year ago.
"I respect a lot what she has done and what she's doing", said Halep, who, like Williams, used to be ranked No. 1. "It is exciting and I am looking forward to it already".