Pliskova had never made it past the third round at a major tournament before but the Czech continued her excellent run at Flushing Meadows with a 6-2 6-2 win.
Pliskova arrived in New York in strong form, having just beaten world number two Angelique Kerber to claim the title in Cincinnati last month.
Her victory over 18-year-old Konjuh was her 10th in a row and took just 57 minutes.
“It’s an amazing feeling,” Pliskova said on court after the match.
“I am so excited to be in my first semi-final, especially in this tournament. I think I played a very good match today.”
“I very much love New York and hardcourt season,” said Pliskova.
“I did a quick break in the first game which helped me and I was more calm then and going for my shots.”
It was a ruthless and dominating performance from Pliskova, who did not face a break point while surrendering only five points on her serve.
The big-hitting Czech, who has fired more aces this season than any other player, opened the match with a bang, breaking the unsteady teen on her opening two serves en route to a 4-0 lead.
Pliskova lost only one point on serve in the opening set as she swept past the reeling Croatian in 28 minutes.
Konjuh began to find some rhythm and confidence near the end of the first and start of the second but could not keep it going and again found herself down a break 3-2.
At that point there was no way back for the 92nd ranked Croatian as Pliskova steamrolled through four straight games finishing her opponent off with back-to-back aces.
“I wasn’t at my best level today for sure, but she was just too good,” said Konjuh, who upset fourth seed Agnieszka Radwanska to reach the quarter-finals.
“Her serve is just too good.
“Today on the court, you know, she found her rhythm and took the opportunities that I gave her.
“This is a great result for me. Now I know that I got that something to be in the top.”
Meanwhile the Tennis Integrity Unit (TIU) is looking into a first-round match at the US Open after irregular betting patterns were detected.
The alert came after 15th seed Timea Bacsinszky’s 6-1 6-1 victory over Vitalia Diatchenko of Russia on Aug. 30.
“As with all cases, the TIU will assess, make a judgement and take appropriate action on the alert information received and obtained for the first round singles match between Vitalia Diatchenko and Timea Bacsinszky played at the US Open on 30th August,” the TIU said in a statement last night.
The TIU added that an alert on its own is not evidence of match-fixing and there can be other reasons for unusual betting including “incorrect odds-setting, well-informed betting, player fitness, fatigue and form as well as playing conditions and personal circumstances that may be relevant”.