The American swimmer claimed victory in the 200-meter butterfly, his signature event, and in the 4x200 freestyle relay
Michael Phelps is building a legacy that looks almost insurmountable at this stage. Last night, the American claimed his 20th and 21st gold medals.
The 31-year-old cut a figure of determination in the early hours of the morning Irish time and looked in the zone as he prepared for the semi-final of his signature event, the 200-meter butterfly.
You underestimate the power of the Dark Side... pic.twitter.com/0exveKlZvn— Deadspin (@Deadspin) August 9, 2016
He went on to claim victory in the final of the event, where he set the world record in 2009, touching the wall in 1 minute 53.36 seconds. Japan’s Masato Sakai took silver in 1:53.40.
He secured his 20th gold medal before holding his arms aloft from the water, beckoning all comers to try and claim his crown as the most decorated Olympian of all time.
And he wasn't finished there.
Michael Phelps claimed two gold medals in the early hours of Wednesday morning Irish time. Image: Michael Sohn / AP/Press Association Images
The Maryland native then played an integral part in the 4x200 freestyle relay as he anchored his team of Ryan Lochte, Conor Dwyer and Townley Haas.
The US 4x200 freestyle relay who claimed gold last night in Rio. Image: Michael Sohn / AP/Press Association Images
And while the news of another gold medal for Phelps will come as no shock to the global audience looking on, it was the fortunes of another American that sent ripples through the Games.
After exiting the doubles tournament at an early stage alongside sister Venus, Serena Williams was unceremoniously beaten last night and dethroned as Olympic champion.
The World number one was beaten in straight sets by Ukraine's Elina Svitolina. Williams had never lost to the 21-year-old in their four previous meetings.
"It's obviously disappointing but, you know, she played really well and I think the better player won today," Williams told CNN.
"I can't wait for the next time and I know next time it's going to be a really good match. I'll look forward to it.
"It didn't work out the way I wanted to, but at least I was able to make it to Rio. That was one of my goals."
Elsewhere last night, Great Britain's Siobhan-Marie O'Connor won the silver medal in the 200-metres individual medley.
Team GB also took silver in the men's 200-metres freestyle relay.
There was also some less heartening news coming out of Rio overnight. A bus carrying media at the Rio Olympics had two windows smashed while travelling - but it's not clear what caused it.
No-one was seriously injured. A photographer working for the Press Association was on board said there were "pierced holes" in the glass when it happened.
Media were being taken from the Deodoro zone back to the Olympic Park.
In the diving pool, meanwhile, the water turned bright green overnight. Some theories point out that it could have been algae or oxidised copper which caused the shift. Olympic officials were quick to issue an update on the water's safety.
The water quality at the Maria Lenk Aquatics Centre was tested and there were no risks for the athletes. We are investigating the cause.— Rio 2016 (@Rio2016_en) August 9, 2016
Green water at the Maria Lenk Aquatics Centre on the fourth day of the Rio Olympic Games, Brazil. Image: Mike Egerton / PA Wire/Press Association Images