Andy Murray beat Juan Martin del Potro on Sunday to become the first tennis player to win two Olympic gold medals in the men's singles.
Their epic four-hour-long match in Rio de Janeiro pushed the two athletes to their limits.
"I'm so tired," Murray told journalists after winning the tournament 7-5, 4-6, 6-2, 7-5.
"Tonight is one of the hardest matches I've had to play for a big, big title," said the British Wimbledon champion, who won gold at the London Olympics four years ago. "Emotionally it was tough, physically it was hard, there was so many up and downs in the match ... Tonight, anything could have happened."
Del Potro said he too was exhausted and had felt like throwing-up during the tournament, but kept going thanks to the support of raucous fans in the stadium.
"After this, I think I have enough tennis for a while," the Argentinian told journalists.
After several wrist operations, del Potro is ranked 141st in the world, but bounced back in Rio to beat number one Novak Djokovic of Serbia and Spanish great Rafael Nadal to reach the final.
"He deserves a lot of credit ... and he should be very proud," Murray said. "If he stays fit, there's no reason why he can't be up at the top of the game."
"Tennis is my life. I suffered when I couldn't play," del Potro said. "Now tennis makes me very happy again."
The two men were relatively evenly matched at the start of their clash, with the scrappier Del Potro capitalizing on the mistakes of the more polished Murray. The Scot expressed frustration, particularly with the fact that his serving had not been up to standard.
Del Potro faded in the third set, showing signs of fatigue and thigh pain. He then pulled himself back up in the fourth and put up a fight until the end, but having missed a chance to serve out the set, lost the test of endurance.
At the end of the match, the two men gave each other a long hug over the net, with Murray patting del Potro on the back, before both collapsing into their chairs. Del Potro could be seen wiping away tears.
Murray now moves on to the Cincinnati Masters. The champion expressed regret that he will not get any time to enjoy his win "because I'm playing a match in 48 hours unfortunately."
Argentina fans made up a big part of the crowd and accompanied del Potro throughout the match with cheers, chanting and boos for his opponent - an unusual atmosphere at traditionally more reserved tennis matches.
One spectator was heard crying out "Papa Francesco" during the match, seemingly invoking divine help from the Argentina-born pope.
Officials repeatedly hushed the crowd to get them to be silent during play and one unruly Argentina fan was escorted out by military officers towards the end of the match.
However, both Murray and del Potro said they liked the atmosphere, with the Argentinian crediting his Rio wins to it.
"The crowd made me cry every night. I think that was the key to keep going and never give up," del Potro said. "It was like a dream."
The marathon match had been preceded by more Rio tennis upsets earlier in the day, with Venus Williams missing her last chance at a Rio gold medal and Nadal failing to win bronze.
Williams and her partner Rajeev Ram took silver in an all-American mixed doubles final against Bethanie Mattek-Sands and Jack Sock. Williams has now matched the five-medal Olympic record in tennis, held by Britain's Kitty McKane.
"It's surreal that I even came out with any hardware at all, it's so well contested here," Williams said.
World number five Nadal, meanwhile, was beaten by Japan's Kei Nishikori in the men's singles match for bronze.
"I didn't play with the right energy during the whole match. I was too tired," Nadal said afterwards.
Swiss tennis veteran Martina Hingis secured her first Olympic medal after two decades in the sport, but had to settle for silver after a women's doubles loss to Russia.