"I hate losing. That's no secret", the 23-times major victor said after digging deep to deny Camila Giorgi a famous victory with a rousing 3-6 6-3 6-4 quarter-final comeback win over the unseeded Italian.
Up next for the 23-time Grand Slam victor is German 13th seed Julia Goerges, who also overcame an early blip in her quarter-final match to defeat close friend Kiki Bertens 3-6 7-5 6-1.
Williams is the first woman since 2013 to reach a Grand Slam semi-final without facing a top-40 ranked opponent.
Kerber is seeded 11th and, after a week of shock defeats for her title rivals, that makes her the highest ranked woman left in the draw.
I am fighting and it helps me gain more confidence with it.
It will be Kerber's first Grand Slam final since she won the second of her two major titles at the 2016 US Open. "She (Ostapenko) won a Grand Slam, as well".
She and Ostapenko have never played each other before and it should be a classic encounter between the German counter-puncher and all-out attack of the Latvian. "It's really important in life, no matter what you're going through, to never quit", Williams said afterwards.
Yet to drop a set, she credits her grass court preparation at Eastbourne the week before the Championships began for getting her ready: "I just practiced really well before Eastbourne, then played Eastbourne and came here". My coach wants me to go for the shots. "I have all this motivation to just win another one".
With Kerber serving for a place in the last four, Kasatkina earned two break points with a forehand and levelled the scores before again failing to hold her serve and being left on the backfoot.
The first game, served by Ostapenko, epitomised the match and illustrated just why the 21-year-old has become such a "must see" player on the WTA Tour and why watching American great Billie Jean King describes her style as "so much fun".
But Ostapenko won the next game to love to move one away and, after another challenging hold for the Slovakian, a backhand victor wrapped up a maiden victory over Cibulkova for the 12th seed. In the second set, the two women played some extraordinary points, and Centre Court was abuzz with what they saw.
The seven-time champion, who missed the tournament a year ago, was outplayed by Italian Camila Giorgi in the first set but rallied to win 3-6 6-3 6-4 on Centre Court.
"She's also kind of aggressive player".
"I don't think Kasatkina has been disciplined enough".
Standing in the way yet could be fourth-seeded Juan Martin del Potro in the quarterfinals - the Argentine's fourth-round match was suspended with him holding a 2-1 set lead - and then possibly three-time champion Djokovic in the semifinals.