In retrospect, it seemed inevitable: Michael Phelps displaying that famous crooked grin and yet another Olympic gold medal, the 19th of a life that progressed, from the world's perspective, through astonishing feats in the swimming pool every four years. But in the eternal second after Phelps began his dive to follow Caleb Dressel in the second leg of the men's 4x100 metre freestyle last night, just before he hit the water, there were no guarantees.

America's capacity for old- world superpower displays has not abandoned them when it comes to swimming but four years ago, they finished second in this race in London. And this was almost certain to be Phelps's last team race. The Baltimore Bullet is 31 now: an old man in a watery theatre starring teenage prodigies.

His fiancee, Nicole Johnson, and three-month-old son, Boomer, were in the stands: he had come full circle from the 15- year-old who made his first Olympic appearance in Sydney, when the world was younger and Dressel was just four years old. Cheering "It was crazy," Phelps said afterwards of the seconds waiting on the blocks while Dressel sprinted towards him, knowing the Brazilians held a narrow lead.

“I honestly thought my heart was going to explode out of my chest. Having that amount of excitement, cheering in the stands during that race: I don’t know if I have heard anything like it.”

It is true. The din in the Aquatic centre last night was lunatic by the time the showpiece race took place. Earlier, everyone watched in astonishment as Katie Ledecky, the iron-minded phenomenon from Washington, chewed up the best efforts of every woman who had ever swum a 400 metres freestyle final by obliterating her own record by a full two seconds. And the presence of Yulia Effimova, one of the Russian athletes competing in Rio despite being initially banned for testing positive for anabolic steroids, reintroduced a retro mood of East-West tension. So it was a night of heightened tensions and Phelps, the master of these evenings, promptly swam the fastest 100 metre freestyle leg of his career.

“The only thing I said to them was: ‘I am going to try and get you guys as much open water as I can.’ I know how hard it is to swim in the wake in that race.”

He had regained the lead by the time he surfaced after the distinctive powerful kick-off which set Phelps apart as an amphibious marvel: a new species of swimmer. By the time he touched the wall to release Ryan Held, he had completed the fastest split of his life, 47.12 seconds. The rest was cheerleading by the poolside.

Phelps coasted through the butterfly 200m yesterday afternoon and is scheduled for the individual medley over the coming days but if yesterday morning proved to be his encore, it was perfect. He has been transformed over the Olympic years from an awkward teenager trapped in the body of an athletic phenomenon into an assured champion of his sport.

A dark undercurrent has run through the first few days of the swimming finals in which the Russian team are challenging for medals under a cloud of acrimony. There were questions over swimmers in Phelps’s first Olympics and they are still present as he prepares to take his leave.

“We all want clean sports,” he said shortly after arriving in Rio.

“I want everyone to be on the same playing field. I am the only one who can control myself and that’s all I focus on . . . in my career, I don’t know if I’ve ever competed in a clean sport. It’s upsetting but there’s not really a lot I can control but me. I agree things need to change – in all sports. It seems to come up at every Olympics and it’s sad that someone in charge cannot control this.”

He can only control his own contribution and he did that in magisterial fashion close to midnight local time last night. He was supposed to be retired. He could easily have been watching this final from his sofa or as a television guest but here he was. No guarantees as he propelled that long frame of his towards the water and even if he hadn’t won, he felt sure there would have been no regrets.

“No, because I’m having fun again. I’m enjoying what I’m doing again. . . whatever’s left, I’ll be able to turn the page and say I was able to finish the way I wanted to.”

With gold medal number 19. As an old-stager, Phelps was able to help his younger team mates respond to the emotion surging through them.

“I started crying. I told them it’s okay to sing, it’s okay to cry . . . These guys are awesome. They will be there in four years – you got this, I am out.”

Exit, pursued by gold.