Anthony Joshua’s career rebuild suffered a devastating setback after he collapsed to a brutal fifth-round knockout by Daniel Dubois at Wembley.
Fighting with the aim of setting up a clash with the winner of Tyson Fury's rematch with Oleksandr Usyk in December, Joshua was instead dominated from start to finish and was floored four times.
Dubois spoke of his determination to "legitimise" his reign as IBF champion after being given the belt which Usyk vacated in June and he did exactly that with a destructive display that could signal the end of Joshua's career.
The 27-year-old underdog went hunting for Joshua from the start and was rewarded as his big shots wobbled the Olympic gold medallist time and again.
An action-packed fifth round of an breathless clash saw Dubois land the decisive blow having come through worrying moments himself as Joshua finally found his distance.
Once Oasis' Liam Gallagher had warmed up the crowd with a live set of three songs, the British rivals engaged in a dramatic opening round that ended with Joshua being saved by the bell.
Dubois, weighing a career-high 17st 10lbs, was on the front foot and looking to land the more purposeful shots, his aggressive use of the head resulting in two warnings.
But at the end of the round he caught Joshua with a savage overhand right that floored the challenger and just as he returned to his feet, the bell sounded.
It was a dramatic start and the fireworks continued into the second round which saw Joshua in full survival mode, desperately clinging on to the underdog as he looked to find the finishing shot.
Dubois was guilty of being too frantic in his eagerness to end the night early but in the third round he had Joshua down again, a left hook causing his legs to buckle beneath him.
A standing eight count was followed by the bell and as the fourth round got underway Joshua was floored for the third time, although he protested that it was not a legitimate knockdown.
Eddie Hearn, Joshua's promoter, looked shell shocked at ringside and his worst fears were realised in an extraordinary fifth.
Dubois was momentarily stunned by two right and just as Joshua looked to pile on the pressure, the 34-year-old was caught by successive right hands himself and he was down again, this time unable to beat the count.
Both fighters entered the ring intent on earning a shot at becoming undisputed world champion against Fury or Usyk having rebuilt their reputations in recent fights.
Joshua had shown in impressive wins against Otto Wallin and Francis Ngannou that the confidence damaged by his shock defeat to Andy Ruiz and two losses to Usyk had returned.
Dubois, meanwhile, was accused of quitting against Joe Joyce and Usyk but he responded by stopping Jarrell Miller and Filip Hrgovic in determined wins full of heart and skill.
With each fighter possessing single shot knockout power, the main event that Joshua entered as clear favourite was not expected to go the distance.
Dubois had the tools to end Joshua's five-fight winning streak, but in his recent appearances had also proved vulnerable to the right hand – Joshua's most effective shot.
But at 27-years-old, he was also an athlete on the rise with greater potential to bounce back from defeat given Joshua was entering the final phase of his career.
How the less experienced fighter – 23 contests compared to 31 – would cope with the size of the occasion and the tension of fight week, including all of its media demands, was an unknown that could only be answered on the night.
And he answered it in emphatic fashion with a stunning performance and finish that puts him on course for a shot at the sport's ultimate prize. Joshua, meanwhile, will struggle to find a way back from such a comprehensive loss.
