One punch can change everything in boxing - especially in the heavyweight division.
The fight isn't over until it's over and below, Planet Sport looks at five of the biggest upsets in the most lucrative, heaviest weight class.
Mike Tyson vs Buster Douglas (February 11, 1990)
Tyson had an agreement to face Evander Holyfield next. All he had to do was beat little-known Buster Douglas in Tokyo, Japan.
At the time, Tyson was undefeated and heavyweight's undisputed champion while his opponent was ranked number seven by the Ring.
Another 90-second obliteration was expected for Tyson, according to analyst Larry Merchant but what we got was a piece of history which will never be forgotten.
In round eight, Douglas tasted the deck but would go on to bounce back and knock Tyson out in round 10 to deliver one of the biggest upsets in boxing history.
An uppercut followed by four punches to the head saw Tyson fall to the canvas for the first time in his career and he was unable to beat the 10-count.
Michael Moorer vs George Foreman (November 5, 1994)
On April 22, 1994, Moorer delivered an upset himself by dethroning Evander Holyfield to win the WBA and IBF titles.
Under Teddy Atlas, Moorer made a first defence against the great George Foreman. It was the same Foreman who fell short against Muhammad Ali in 1974.
Foreman had tasted defeat to Tommy Morrison in the previous fight but what we were about to witness was a piece of boxing history.
Untested and in control for the opening nine rounds, it appeared to be a routine win for Moorer. That was until round 10. Foreman came out and threw a flurry of punches before landing a short right hand to knock Moorer to the floor.
The American never got up and it saw Foreman crowned as heavyweight champion only for the second time in his career. This time, he was 45 years old and it made him the oldest heavyweight champion of all-time. That record still stands.
Lennox Lewis vs Hasim Rahman (April 22, 2001)
Following three world title defences, Lennox Lewis was plotting a showdown against long-time rival Mike Tyson.
The fight would take place in South Africa and Lewis did not arrive in South Africa until 12 days before the fight. Compared to his normal fighting venue in Las Vegas at 2000 feet above sea level, South Africa was more than double at 5,200.
It proved to be a huge factor on fight night as huge underdog Rahman delivered an upset with a fifth-round KO. The pair would rematch after Lewis won a court battle to get an instant second fight.
The fight took place back in Las Vegas and this time, Lewis avenged the defeat with a fourth-round stoppage.
Wladimir Klitschko vs Tyson Fury (November 28, 2015)
Unbeaten in 11 years, and the longest cumulative reign as heavyweight champion at 4,382 days, not many gave Fury a chance of beating Dr. Steelhammer.
And to make things even tougher, Fury was fighting in Germany - a country where Klitschko had become a huge superstar at.
However, the fight itself saw Fury somehow overcome everything against him and outpoint Klitschko to win the WBA, IBF, WBO and IBO titles. The win was not only an upset, but a result which changed the course of the heavyweight division for years to come.
Anthony Joshua vs Andy Ruiz Jr (June 1, 2019)
Ruiz stepped in as a late replacement for Jarrell Miller for Anthony Joshua's American debut at Madison Square Garden.
This was supposed to be a gimme fight for Joshua, who wanted to establish himself to the US audience.
And it started well for Joshua who dropped the Mexican-American in the third round but moments later, the Brit also tasted the canvas. He was down again before the end of the round and had to rally back in the next few rounds despite being visibly hurt.
A further two knockdowns happened in round seven and the referee had no other choice but to halt the fight. Ruiz's famous win is regarded as the biggest upset since Lewis-Rahman.
The pair would rematch later that year in December but Joshua would regain his WBA, IBF, WBO and IBO titles with a points win in Saudi Arabia.