Even the best race of his Olympic-resume-building career couldn’t bring Arman Hall back-to-back world 400 meter titles.

The reigning IAAF World Youth champion set a personal-best 45.39 to finish second in the 400 meters at the IAAF World Junior Track and Field Championships at the Montjuïc Olympic Stadium in Barcelona, Spain on Thursday night.

For the time being at least, the title of world’s premier junior quarter miler belongs to the Dominican Republic’s Luguelin Santos, who validated his world-leading status by winning in 44.85.

Although it was a bittersweet moment for Hall, the former St. Thomas Aquinas prep All-American headed for the University of Florida proved once again he lives for the big races.

“The turning point of the race was the last 200 meters,’’ Hall said. “It felt great setting a new personal record. I was not trying to think about what time i wanted to run. I was just thinking about running the best race I can.”

Most pre-race projections had Aldrich Bailey, Hall’s more heralded United States teammate and ranked No. 2 in the world (45.19), providing the stiffest challenge to Santos, who appears to be following in the footsteps of Felix Sanchez as the next great Dominican quarter miler.

Hall changed all that, however, by making his move at the 250-meter mark and then holding off a late charge from Bailey and Australia’s Steven Solomon, who finished tied for third in 45.52.

After clocking a previous personal-best 45.59 to win the Great Southwest Classic in June, which at the time ranked No. 2 all-time in Florida prep history, Hall has now eclipsed the 45.44 state prep record set by Xavier Carter.

Hall now shifts his attention to helping the United States win gold in the 400-meter relay, which starts Friday and then the 1,600-meter relay.

Meanwhile, Robin Reynolds, Hall’s future University of Florida teammate, had a forgettable day in the long jump.

A year after finishing fourth in the 400 meters at the IAAF World Youth Championships, Reynolds, the most decorated track and field athlete in Florida high school history, failed to make it to the finals.

Reynolds could only muster a leap of 20-1 to finish 10th in the second flight. It was well off the personal-best 20-6.25 Reynolds cleared to set a Florida state prep record on April 28.