RIO DE JANEIRO: At his fifth, and supposedly last Olympics, Michael Phelps finally let his guard down, sobbing along with his team mates after collecting his 19th gold medal by helping the United States win the men's 4x100m freestyle relay in Rio.

With his infant son Boomer watching from the stands, cradled in the arms of his mother, Phelps's fiancee Nicole Johnson, the greatest Olympian of all time began to weep after two of his teammates, Ryan Held and Caeleb Dressel, started to bawl.

“They were making us cry,” Phelps said.

“The younger guys started crying, I started crying.”

Already far and away the most decorated Olympian of all time, the 31-year-old Phelps showed that he had lost none of his speed or killer instincts after coming out of a brief retirement to provide Team USA with its first win in the event since Beijing, narrowly holding off France and Australia.

“It was crazy. I was standing on the block while Caeleb was coming in and I honestly thought my heart was going to explode out of my chest,” said Phelps, who clocked 47.12 seconds on the second leg to give the US swimmers a lead they never relinquished.

“Having the amount of excitement, cheering that was in the stands tonight during that race - I don't know if I've heard anything like it,” Phelps said.

“We wanted that race back so badly,” he added. “My last 400 free relay ever, it feels damn good to get a win.” Nathan Adrian, the 100m individual winner in London four years ago, swam the anchor leg in a sizzling 46.97, the fastest split of the race, to earn his fourth career gold medal, spread over three Olympics.

“Like seriously, I've won gold in the 100 freestyle but I dream about the 4x100 freestyle relay and there it is,” Adrian said.

“You've got to enjoy this moment, you've got to take it in for what it is.”

Winning the relay had become a matter of national pride for the Americans after they were beaten in London in 2012, and failed to make the final at last year's world championships in Russia.

Nothing was left to chance, with the team even calling in four-time NBA champion Tony Parker to talk with the swimmers during one of their final training camps.

“I love a little emotion, I had to fight back some tears myself. That's the one you grow up as a kid, a young swimmer dreaming about it,” Adrian said.

“We actually had Tony Parker come in, he's half French, half American guy, when we were in training camp, and he says it best, the second championship is the one that's the hardest and the one you appreciate the most.”