Park will face off against 2004 Curtis Cup teammate Sarah Huarte, who downed Morgan Pressel 1-up. Creamer meanwhile will square off with Amanda McCurdy, who claimed a 3-and-2 decision over Sun-Young Yoo.
Yoo's loss creates the first all-American semifinal since 1994.
The 17-year-old Park grabbed control of the match midway through the front nine, thanks to Allen's troubles. Allen stumbled to three straight bogeys from the fifth to fall 3-down.
The duo remained tied there as they matched one birdie and four pars over the next five holes. Allen tripped to another bogey at the par-4 13th to fall 4-down. Allen came right back to birdie the very next hole to get back within 3-down.
Park, who shot even-par for her round, claimed the 4-and-2 win as Allen bogeyed the 16th hole.
'This is the biggest field for the amateurs and the strongest field,' said Park, last year's runner-up. 'Three Curtis Cuppers in the semifinals really says something about our team and how strong we were. I know what it takes, it takes a lot of energy. This tournament is all about endurance and mentality.'
Huarte trailed for eight holes and didn't take her first lead until the 17th hole. Pressel quickly moved 2-up with birdies at the second and fourth. Huarte began to fight back into the match as Pressel bogeyed the seventh.
Huarte, the 2004 NCAA Individual champion, rolled home an eagle from 5 feet out at the par-5 eighth to square the match. Each player parred the next six holes to remain tied.
Pressel regained a 1-up lead with a birdie at the par-3 15th. Huarte birdied the next to square the match, then took her first lead when she made a tough up-and-down par and Pressel bogeyed the 17th. The duo halved the last with pars to give Huarte her first trip to the semifinals.
'My heart was pounding over that chip,' Huarte said of her shot on 17. 'I felt it. And over the putt and the next four shots. Gosh, that was a pretty clutch shot. That was just a big hole there.'
Creamer never trailed in her match. She birdied the second to move 1-up. Ramage squared the match with a birdie on the par-4 sixth. Ramage then bogeyed the seventh and Creamer eagled the eighth to grab a 2-up cushion.
Creamer stumbled to a bogey at No. 9 as her lead dropped to 1-up. Ramage, who had won her previous three matches in 19 holes, bogeyed the next and Creamer birdied the par-5 12th to claim a 3-up lead. The duo matched pars on the next four holes to give Creamer the match.
'I'm just feeling really confident, very comfortable out here,' said Creamer. 'I really like the golf course, especially when your short game is good, knowing that my speed control's been very good.'
McCurdy, like Creamer, never trailed and she also owned the largest cushion of the day. Yoo bogeyed the second and McCurdy birdied the third to take a 2-up lead.
McCurdy, who is playing in her first U.S. Women's Amateur, dropped in a birdie at the fifth to move 3-up. Yoo then bogeyed the seventh, before McCurdy birdied the eighth to take a commanding 5-up lead.
The 20-year-old Curtis Cupper padded her lead with a birdie at the 10th. Yoo then mounted a comeback. She birdied the 12th and got it back to 4-down as McCurdy bogeyed the 13th.
McCurdy tripped to another birdie on No. 14 as her lead slid to 3-up. The pair parred the next two holes to give McCurdy the match.
'I had given myself enough of a cushion that I was probably going to be okay,' said McCurdy, a junior-to-be at the University of Arkansas. 'At some point I was probably going to tie her on a hole again or beat her.'
The semifinals are slated for Saturday, with the 36-hole final scheduled for Sunday.