The 37-year-old American beats the French Open champion 6-3 7-5 on Centre Court.
Venus Williams can feel sister Serena fighting alongside her as she tries to keep the Wimbledon title in the family.
The sisters have won seven of the last 10 titles between them at the All England Club, with Serena claiming five and Venus two.
But Serena was unable to compete for her third success in a row as she awaits the birth of her first child back home in Florida.
Also absent is the sisters’ father Richard, whose dream it was to see them winning grand slams.
“They’re definitely here with me, for sure,” said Venus after a 6-3 7-5 win over French Open champion Jelena Ostapenko in the quarter-finals.
“Even if it’s not physically. That is one thing I do know. They’re fighting right alongside me.”
Ostapenko was only a few weeks old when Williams, the oldest semi-finalist since Martina Navratilova in 1994, made her Wimbledon debut 20 years ago.
The 37-year-old has not won a grand slam crown since lifting the Venus Rosewater Dish for the fifth time nine years ago.
But she has been getting closer, reaching the semi-finals here 12 months ago and then losing to sister Serena in the final of the Australian Open in January.
Should she win the title on Saturday, Venus would overtake her sister as the oldest female grand slam singles champion in the open era.
“I love it,” she said. “I try really hard. There’s no other explanation. You do your best while you can. That’s what I’m doing.
“I love the challenge. I love pressure. It’s not always easy dealing with the pressure. There’s constant pressure. It’s only yourself who can have the answer for that.
“I love the last day you play, you’re still improving. It’s not something that is stagnant. The competition keeps you growing. You have to get better if you want to stay relevant.”
Williams’ longevity has been accentuated by the age of her Wimbledon opponents, who have also included two 19-year-olds and a 21-year-old.
Williams won the first three games and held on to her advantage to claim the opening set before moving 3-1 ahead in the second.
Back came Ostapenko but the Latvian could not find the right balance of winners and errors, and Williams won the final three games.
“I know she had to be feeling confident,” said Williams. “She played a great match. Not a lot of errors.
“I’d never played her, watched her, didn’t really know what to expect. The grass, of course, changes the game. I was really happy to come out on top.”
Garbine Muguruza moved one step closer to winning her second grand slam title by beating Svetlana Kuznetsova in the quarter-finals.
The Spaniard was a runner-up at the All England Club in 2015 but looks primed to go one better this year after easing past the experienced Kuznetsova 6-3 6-4.
Kuznetsova enjoyed strong starts in both sets but failed to capitalise and it was Muguruza’s superior serving and movement that proved decisive on Court One.
“I’m very happy and very pleased also with this match, because obviously Kuznetsova is a very tough opponent,” Muguruza said. “We all know she has been and is a great player. I managed to play a good level during all the match. I earned the victory.”
Rybarikova becomes the latest outsider to make a run to the last four of a slam, with the world number 87 defeating Coco Vandeweghe 6-3 6-3.
The Slovakian upset title favourite Pliskova in round two and has kept her remarkable run going.
She was in control of the match at a set and 2-2 when heavy rain prevented any further play on Court One.
The pair moved under the roof on Centre Court to complete the contest but it did not disrupt Rybarikova’s momentum.
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