American Caeleb Dressel added a world record to his growing medal tally but he won't be joining swimming's exclusive Olympic club.

Dressel's bid to win six gold medals at the Tokyo Olympics was over before he even dove into the pool for the new 4x100m mixed medley relay.

The Americans were too far behind when their top swimmer took over and the best Dressel could do was rally the US to a fifth-place finish in the race that features two men and two women on each team.

Before the relay, Dressel set a world record while winning his third gold of the games in the 100m butterfly.

Dressel led right from the start and held off Hungary's Kristof Milak to touch in 49.45 seconds, breaking the mark of 49.50 that the American set at the 2019 world championships.

Milak, winner of the 200m fly, earned the silver with a blistering 49.68. The bronze went to Switzerland's Noe Ponti.

When he saw the "WR" beside his name, Dressel smiled and joined hands with Milak in the lane next to him.

They raised their arms together before Dressel flexed his left arm and pumped it in the air.

"What a close race and two of the fastest times in history," Dressel said. "You don't get that very often, so to be part of that is really special."