After years of yearning for a fight against Amir Khan, Kell Brook realised that in the afterglow of the most satisfying win of his career there was nothing else to fight for.
In hanging up the gloves aged 36, Brook walks away from potential lucrative fights against welterweight contender Conor Benn and middleweight contender Chris Eubank Jr.
Why would Brook (40-3, 28 KOs), from Sheffield, England, retire now with money-spinning options for future fights available, and after such a good performance in battering Khan to a sixth-round stoppage just three months ago?
There are a few reasons why Brook decided to end it now.
Firstly, for Brook nothing can beat the moment he overpowered Khan in February to settle their festering feud. After six years of trash talk and attempts to make the fight happen, Brook produced the performance he had dreamed of.
Khan and Brook, then 28 and living just 50 miles apart in rival northern English counties of Lancashire and Yorkshire, both entered the welterweight class in 2014. For Brook, beating Khan -- after almost a decade of animosity between them -- was perhaps the sweetest moment of Brook's career, one that could not be bettered and ended with respect between the pair.
It was the best display of Brook's career except for the points win over Shawn Porter to win the IBF world welterweight title in the United States in 2014.
After conquering Khan, reigning as world champion and losing to some of the best boxers in his era, Brook's motivation had run out. As for money, Brook is settled after banking millions including at least £3 million from fighting Khan in Manchester in February.
"My heart's no longer in it, I've got nothing left to prove," he told the Telegraph newspaper. "It's not there anymore.
"I needed the Khan fight, I needed to settle the grudge, the feud. There is no dark feeling left in me now, I think when you have been in the ring with someone it passes, it leaves you. Me and Amir said some words which were hateful in the build-up, but that's what happens in boxing. But I respect him after the fight. He showed real heart in there."
After the fight, there was weighted expectation that one or both boxers would announce their retirement. Khan told reporters afterwards: "In the ring, I didn't have excitement and that push. That's maybe a sign that I should call it a day, but let's see." Yet in the days that followed neither side made a public decision and only the hints remained.
The second point is that Brook was locked out of the world title picture. After reigning as IBF world welterweight champion for three defences, Brook lost the belt when Errol Spence knocked him out in round 11 in 2017.
Spence, the IBF-WBC-WBA champion, and fellow American Terence Crawford, the WBO titleholder, are the best in the division and Brook has emphatically lost to both. Crawford took Brook apart in four rounds in November 2020 and a world title shot at welterweight was not likely as he was not ranked in the any of the four world governing bodies' top ten, no matter how impressive the Khan win was.
After overwhelming Khan, Brook said at the post-fight news conference: "I've had a fantastic career and been over to America to win a world title, been in with Gennadiy Golovkin, Errol Spence Jr. and Terence Crawford, but I always wanted to have this fight with Amir. I felt like a 22-year-old in the ring today, but I'm going to sit down with my family and talk about it."
Retirement was on Brook's mind the moment he finished off Khan.
Thirdly (and who could argue against this?), Brook had to consider his health after an 18-year professional career which looked in jeopardy of being cut short when he suffered broken eye sockets in stoppage defeats against Gennadiy Golovkin and Spence in 2016 and 2017.
Brook suffered a further stoppage at the hands of Crawford in November 2020 and he will have asked himself over the last three months: is it worth suffering another punishing stoppage loss?
It was a good call from Brook, who will be remembered for more than just his rivalry with Khan. He fought (and lost) to currently the best two welterweights and two of the best boxers in any weight class, and even stepped up two weight classes to middleweight to face Golovkin, who at the time was arguably the best pound-for-pound fighter in the world.
Brook also produced one of the best performances by a British boxer on U.S. soil when he earned a majority decision over Porter in Carson in August 2014.
But the Sheffield boxer found himself in hospital fearing he would never box again after he was stabbed in the leg while on holiday in Tenerife two weeks after he beat Porter.
That episode was typical of Brook's roller-coaster career, which swung from moments of triumph, to career threatening injuries, to huge nights against the biggest names, to knock-out defeats.
After the losses and eye injuries in 2016 and 2017, it took Brook three years to earn another world title shot. Following defeat to Crawford, many fancied Khan's speed to unravel Brook, but the Sheffield boxer's strength, desire and conditioning were too much for his rival.
Khan (34-6, 21 KOs), 35, must now be wondering if he should follow Brook, who has delivered the perfectly timed parting shot to a brilliantly entertaining career.