Olympic champion Jeremy Wariner produced a perfect 400m race to take gold in Helsinki while Britain's Tim Benjamin came a creditable fifth.
The 21-year-old American was already well ahead coming off the bend and eased away to win in 43.93 seconds - the seventh fastest-ever time.
Team-mate Andrew Rock claimed silver in a personal best time of 44.35 while Canada's Tyler Christopher took bronze.
A high quality race saw five athletes dip below the 45-second barrier.
"I've waited to go under 44 seconds for a while and I did it in a great race and against great competition," Wariner told BBC Sport.
"My goal is some day to break Michael Johnson's world record and as I mature it will be possible."
Benjamin only broke the 45-second barrier for the first time earlier this month at Crystal Palace - a race in which he handed a shock defeat to Wariner.
And the 23-year-old Welshman showed his step up to world class by reaching the final and clocking 44.93 from the difficult lane one.
"It wasn't too bad," Benjamin told BBC Sport. "I struggled a bit with lane one and I was out of contention coming off the bend.
"The time is my third best. It was an awesome race and I think I can run a low 44 after a full winter's training.
"You have to give people time to develop and I am definitely on the up and up."