TOKYO, Japan — American Chase Kalisz broke the pain barrier to beat Japan’s Kosuke Hagino and Daiya Seto to gold in the 400-meter individual medley at the Pan Pacific swim championships on Thursday.

The double world champion fired a warning for the 2020 Tokyo Olympics as he schooled the two local favorites to win in four minutes, 7.95 seconds with Rio gold medalist Hagino settling for silver in 4:11.13.

American superstar Katie Ledecky had a mixed first day after romping to gold in the 800-meter freestyle, only to be ambushed by Canada’s teen sensation Taylor Ruck in the 200-meter.

Kalisz, silver medalist at the 2016 Rio Olympics, produced a sinew-shredding third leg of breaststroke to overhaul Hagino and the fast-starting Seto, who finished third in 4:12.60.

“I think it was pretty obvious I was hurting on my freestyle,” Kalisz said after bagging a fifth American gold and dealing a psychological blow to his Japanese rivals two years before the Tokyo Games.

“But it’s a championship-level meet and it’s about getting your hand on the wall first, not so much about the times.”

A deflated Seto admitted: “Kosuke and I went out really hard and just kind of died out there together.”

Olympic champion Ledecky cruised to an easy victory in the 800-meter freestyle, flirting briefly with her own world record before clocking a championship best 8:09.13.

Australia’s Ariarne Titmus, the Commonwealth champion, was a distant second almost eight seconds back with American Leah Smith third in 8:17.21.