In four games against Williams, Strycova has not yet won a set.
The match between Halep and Svitolina was marked by long exchanges, but its consistency, superb two-handed backhand and concentration made the difference.
Svitolina, who played in her first semifinal singles Grand Slam, and Halep have similar styles. Both are fast with balanced basic games. Svitolina had won four of their previous seven games, while Halep won their last duel: an exhausting set in three semifinal heats in Doha this year.
But Halep has pioneered through the tight and tense early matches, and has consistently found a higher speed.
"It's one of the best moments of my life," said Halep.
Halep, former No. 1 champion and the French Open, has now reached the final of three different Grand Slam tournaments: the French Open, the Australian Open and Wimbledon. Svitolina's most important victory was won last season in the WTA final in Singapore.
"I was nervous, but I think it was the same as in other matches, like big matches," said Svitolina. "I played a few, I would say. I did not play as much as she probably did because she won a Grand Slam, so she knows what it's like to get an important result in your career. I do not know if there is a little bit of experience today, but I think it was amazing today. She moved really well and hit the ball perfectly.
Halep, seventh seed, has fresh air in the legs and in mind at Wimbledon this year. After a three-set fight with Romanian compatriot Mihaela Buzarnescu in the second round, she has won her last four matches in two sets.
She defeated former No. 1 Victoria Azarenka in the third round, stopped the race of American Coco Gauff in the fourth round and beat China's Zhang Shuai in the quarterfinals.
In this form and mood, it could be a threat to Williams in Saturday's final. Williams was pissed off in her two major finals last year while she was looking for her 24th title in singles. But she is 9-1 against Halep and won her most recent game, beating Halep in three sets in the round of 16 of this year's Australian Open.