It was Nadal's second match of the day, defeating countryman Albert Ramos-Viñolas earlier in the day.
Pliskova, fighting to hold off Romania's Simona Halep atop the rankings, first ousted Italian qualifier Camila Giorgi 6-3, 4-6, 6-0 in one of five matches postponed from Thursday due to rain.
Despite his semifinal loss, Rafael Nadal will return to No. 1 in the Emirates ATP Rankings on Monday.
Kyrgios, who ousted Monday's new world number one Rafael Nadal in the quarter-finals, has not won an ATP title since taking his third a year ago in Tokyo. But he promptly broke back before closing out the night.
Spekaing after the match, the Spaniard told reporters: "He obviously is a tough match always.Nick is a great player, but I played poorly".
With both men backing up after round of 16 wins earlier in the day, the Australian charged out of the blocks in the first set and was never headed in the 6-2 7-5 victory.
He will next play David Ferrer while American John Isner takes on Bulgarian Grigor Dimitrov on the other semi-final.
"To be No. 1 after all the things I've been going through the last couple years is something unbelievable", Nadal said.
Stabbing spree in Finland leaves two dead, several injured
The country's president, Sauli Niinisto, visited Turku later Friday and condemned what he called a "shocking and cowardly act". The suspect was shot in the leg after the attack, said police, who warned the public to move away from the city center.
In the women's doubles, Taiwan's Hsieh Su-wei also played two matches, winning both.
After having to come through a tie-break in the opening set Nadal breezed through the second to set-up the Kyrgios match.
Kyrgios benefitted from a Ferrer double fault at 1-2 in the first set tiebreak and using his big serve-forehand combo to take the first set 7-6 (3).
"He was a nightmare to play, especially because, you know, I'm a tall guy and he's not the tallest guy", Nick Kyrgios said of his opponent. And I feel good.
A Wimbledon and Australian Open semi-finalist, Dimitrov has excelled in Cincinnati, not dropping a set en route to his 12th ATP World Tour final and third of the year. Neither player converted a service break in the marathon match. "Whatever happens, happens", Kyrgios said. "I recover well, so definitely I will be ready".
"I had the match on my racket and tightened up", Kyrgios said.
With only two hours between matches, the Czech then defeated Denmark's fifth-ranked Caroline Wozniacki 6-2, 6-4 to book a semi-final berth.