Amir Khan suffered his first knockdown at the hands of amateur opponent Craig Watson

A first glimpse of Amir Khan's vulnerability was given by amateur opponent Craig Watson, but what happened next to the man who floored Britain's Olympic hero?

The returning silver medallist had received a standing ovation as he arrived for his opening fight in the England ABA Championships at Preston Guild Hall in February 2005.

Watson was unprepared for such a high-profile bout, having initially thought he would merely appear on the same show as the Athens Olympian. But Khan's late entry into the same weight class set the scene for a flurry of punches that highlighted his defensive frailty.

"It was on my birthday. It's one of the best yet, I'll tell you that for nowt," recalled the Mancunian, back then a trainee steel fixer, who was drawn against Khan, the hottest teenage talent in Britain.

Khan gave an early exhibition of his blurring speed, pinning Watson in the corner as he opened an expected lead on the scorecards.

Watson was edging closer in his southpaw stance, adjusting his range with a few probing punches as he attempted to counter Khan's whirring combinations.

"The left hand that I clipped him with was a good one, a humdinger."

Khan stood bolt upwards for a split second, his body briefly frozen by Watson's left hook, but then his legs dramatically crumpled as he fell backwards onto the canvas.

Pumping his fists in delight, Watson wanted to replace Cuba's Mario Kindelan as the last man to defeat Khan, but believes the 18-year-old was afforded extra recovery time to reorganise a stray shoelace.

Khan ended the fight with a bloodied nose and a 21-9 win. It was Watson who toured ringside like a victor, reliving his defiant blow in media interviews that heralded the end of his unpaid career.

Watson was swiftly summoned to London by promoter Kellie Maloney, formerly Frank, who was eager to tie down the in demand 22-year-old on a professional deal.

"I thought when I turned pro I would have all the backing, all of the money, and all of it going my way, and it wasn't the case," Watson admitted ruefully.

He was pitted against John Fewkes, a more seasoned unbeaten prospect after only a handful of fights, suffering a narrow points loss, and even after becoming a champion, Watson was wracked by financial concerns.

The Commonwealth title holder was grafting in a warehouse, having been 'stupid with money', and a hastily agreed European title fight with Daniele Petrucci ended in a third-round stoppage defeat.

"When I fought the Italian, I was a Commonwealth boxing champion, and I'm thinking, 'why the bleeding hell am I working? I've got no sponsor, I've got nothing.' I'm a champion."

Watson had reverted back to the role of an opponent. He was hand-picked for the opposing corner to Matthew Hatton on the undercard of Ricky Hatton's homecoming fight against Juan Lazcano at City of Manchester Stadium.

A few rounds in, Hatton's family members were becoming increasingly exasperated with Watson.

"I remember during one of the rounds, his dad shouting at him, 'step back and work Matthew, step back and work.' I just said, 'listen to your dad,' trying to goad him."

In replays of the fight, Watson gleefully picks out boxing greats such as Mike Tyson and Oscar De La Hoya watching from ringside before famed MC Michael Buffer announced his wide points win.

No more the 'one-punch wonder,' now a respected champion.

"I think I reached beyond my level, because when I turned pro, I never thought I'd reach anywhere near that."

That grandiose victory along with a British title triumph in a rematch with John O'Donnell are the stand-out achievements in Watson's 27-fight career, featuring 22 wins and five defeats.

A lanky heavyweight prospect had appeared below Watson on the bill for both O'Donnell fights.

"Tyson Fury, the heavyweight champion of the world, boxed on my undercard. It's just crazy isn't it."

There had been fleeting talk of another fight with Khan, who fulfilled his own ambitions of world title glory, and both men were briefly reunited when Watson fought on the same show as the WBA champion's defence against Paul McCloskey at Manchester Arena.

"I always wanted to get Khan in the pros, but he's tried to do lightweight where I was trying to do welterweight, and it was never to be."

Khan still fights on, seven years after Watson left the ring for a final time, and a potential meeting with Manny Pacquiao is the latest tantalising option that appears to be putting off any thoughts of retirement.

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Ever since Watson's perfectly placed left hand, Khan has endured a cluster of knockout losses amongst his numerous title wins, and his old foe offers his own theory about this infirmity.

"Everyone says Khan's chin doesn't work. It's not his chin. I think it's his legs."

Watson is well placed to know. His proud friends will occasionally regale onlookers with the tale of how the boxer from Manchester's Northside club felled Britain's boy wonder.

"I wouldn't say I was the best amateur in the world," says Watson.

But that one punch was enough.