Tunisia's Ahmed Hafnaoui had the swim of a lifetime, pulling off a surprise win in the 400m freestyle. The odds were stacked against the 18 year old as he swam in lane eight and had to fight from the outside to maintain his momentum.

The race was tight for most of the eight laps with Hafnaoui able to stay near the top three throughout. He out-touched Australia's Jack McLoughlin who took home the silver medal. The teenager had a near perfect finish for a time of 3:43:36. Hafnaoui was the youngest swimmer in the field. American Kieran Smith rounded out the podium, and will take home the bronze medal.

Just after finishing, the animated teenager let out a huge scream when he realized he had pulled off the upset. Afterwards he remarked, "I just can't believe it. It's a dream and it became true. It was great. it was my best race ever."

The gold medal is only the fifth by a Tunisian athlete at the Olympics, with three of those medals coming in swimming.

Reuters contributed to this report.