Anthony Joshua acknowledged the fact he had had only 12 days to prepare for Carlos Takam contributed to his struggle to secure the explosive knockout expected of him.
Though he consistently dominated the latest defence of his IBF, IBO and WBA titles, he took until the 10th round to stop the previously little-known Frenchman, and to significant dissatisfaction.
The 36-year-old protested when referee Phil Edwards rescued him amid increasing punishment, Joshua refused to celebrate, and many of the estimated 76,000 present booed when he did so.
Takam's resilience was even more impressive given he had replaced the injured Kubrat Pulev, a bigger and different style of fighter, with so little notice after the Bulgarian's withdrawal through injury.
Despite neither Pulev nor Takam representing an opponent even near the status of Wladimir Klitschko, who Joshua had previously stopped, a world record crowd for an indoor fight had gathered under the roof at Cardiff's Principality Stadium.
It was the entertaining stoppage of Klitschko that had created such high levels of expectation surrounding Joshua, 28, and the champion said: "Against Pulev, I'd have been able to have been a little bit more effective.
"Takam went down in the fourth; maybe Pulev wouldn't have made it through the fourth.
"Takam was shorter, he knows how to ride that right hand; that's what made it interesting with the change of opponent. He deals with guys like me every day in the gym; he's naturally got a shorter style, and I don't deal with guys like Takam every day.
"It was just going to be a little bit more difficult but with him, certain mistakes I could have been making five fights ago I'm not going to make any more against someone like him. All they want to do is land one shot; he knows his durability so wants to stay in the ring for 10 rounds, 11 rounds, and try and land that one sweet haymaker.
"I have to go back to 2014 to think of guys I've sparred who are like Takam; I'm sparring big guys every other week, because there's not so many short heavyweights."
Joshua's trainer Rob McCracken described the challenge presented as "tricky" but despite recognising his fighter is still developing, did not dismiss promoter Eddie Hearn's post-fight talk of Joshua accepting his first international fight in 2018.
"Takam wasn't ideal with 12 days' notice," he said. "We're very professional, and that's not ideal to fight someone on 12 days' notice, that's the long and the short of it.
"It was obviously tricky because Takam's short, squat, awkward, dynamic at times, but Anthony got the job done and found a way to win each round and close the show despite a bad head collision.
"He'll learn from this, get stronger and smarter, and at the end of this journey hopefully be the full package. He's the number one heavyweight in the world and will hopefully cement that over the next year or two, possibly abroad."
Hearn added: "You can't just say 'Cool, Takam instead'. He's been programmed for nine weeks for Pulev; now he's fighting a different style that's never going to suit him."