Each claimed $US10,000 ($NZ12,270) for impressive winning performances in the 22-foot Olympic standard halfpipe.
Rising star Chloe Kim (13) (US) set the early standard in the women's competition with 85.80 points, before Clark took control.
Clark's second run, with a massive frontside air, backside 540, frontside 1080, cab 720 and frontside inverted 720, earned her 89.60 points and top spot.
''I was happy with my second run, but glad I pushed it on the third,'' Clark said.
''It's always fun doing a competition heading into the Olympics. Every four years, the level of riding improves so much. The girls really got into it today and were inspiring each other.''
Kim, clearly one to watch, finished second and Gretchen Bleiler (US), who put together an amazing come-from-behind final run to score 80.60 points, was third.
In the men's competition, Greg Bretz (US) put together a solid first run to take the early lead with 89.40 points.
But Hiraoka's storming second run propelled him into the winning podium position with 94.80 points. He blasted a backside 540, frontside 1260, backside 900, 1080 double cork and cab 1080.
Fellow Japanese rider Ryo Aono also had a strong second run, with a score of 85.00, enough to see him take third place behind Bretz.
''I felt confident and amped going into my second run,'' Hiraoaka said.
''I was stoked to stomp another frontside 1260, only the second time I've done that in competition.''
On Thursday, Stale Sandbech (Norway) and Jamie Anderson (US) claimed the slopestyle titles.
Anderson produced a classy second run to score 85.20 with a backside tailslide, 50/50 on the closeout rail, 50/50 on the rainbow, backside 180 mute, cab 540 melon and backside 540 indy.
''My run was pretty good, but a bit sketchy,'' Anderson said.
''You have to play it out moment by moment. I fell on my first run, but I was glad to land my second, especially in these conditions.''
Enni Rukajarvi (Finland), who had been in the lead until Anderson's fierce run, finished second with 79.20 and Silje Norendal (Norway) was third with 69.20.
Silje snatched the final podium spot from New Zealand rider Rebecca Torr, who finished fourth with 65.20. Stefi Luxton was fifth with 61.80.
Sandbech, who placed third at last year's Burton High Fives slopestyle, nailed his second run to score a winning 94.60 in the men's event.
Kyle Mack (US) was second with 92.40, and Yuki Kadono (Japan) was third with 91.20.
