The diminutive winger sidestepped through a gap in the Wallabies' defence with the clock already in the red, giving him a fairytale finish to his nine years at the top of the game that elicited as big a cheer as has ever been heard at the Cardiff stadium.
The ending, for Williams, was slightly bittersweet since Wales were on the wrong end of a 24-18 defeat - but despite that, the winger's final action in a Wales jersey assured him of a place in the history books as one of the few sportsmen to provide a genuinely unforgettable finale.
He's not the first to have done so, however, because for better or for worse some of the great names in sport have gone out on either unforgettable highs - or equally unforgettable lows.
Here's our pick of the best, but as ever please feel free to leave your own nominations in the comments box down below.
- - - - -Steve Redgrave wins his final ever race, twice
The legendary British rower earns pride of place in the list by having closed his career by winning Olympic gold not once, but twice.
Redgrave defied the effects of both type 1 diabetes and colitis throughout an extraordinary rowing career, winning his first Olympic gold medal in 1984 and taking his fourth 12 years later in Atlanta in what was to be his final race.
"If anyone sees me anywhere near a boat again, they have my permission to shoot me," he famously declared after that fourth gold medal in 1996.
That retirement lasted just four months before he decided to get back into the water. "He has lived with the job so long now he doesn't know any other way," his wife Ann said. "People just can't switch off like that."
Four years later, Redgrave won gold at a fifth successive Games as he took yet another gold in Sydney, cementing his place as one of the greatest Olympians of all time.Zinedine Zidane goes out with a headbutt
The French superstar enjoyed a stellar career, winning league titles with Juventus and Real Madrid, lifting the Champions League trophy, and inspiring France to an incredible World Cup victory in 1998.
Eight years later, deep into extra time in the World Cup final, he looked likely to cap all those glories with a second World Cup victory. The unfancied French side were pushing Italy hard in the final, and Zidane had almost scored a decisive header.
But with 10 minutes left until a shootout, Marco Materazzi called his sister a whore, prompting Zidane to headbutt the Italian in the chest. Materazzi made the most of the blow, falling to the turf as if he'd been bullseyed by a sniper on the roof of Berlin's Olympiastadion, and the referee sent the Frenchman off. France went on to lose 5-3 on penalties.Nigel Mansell quits F1 after finally winning title
The Brummie F1 star had endured a string of miserable misfortunes before finally being crowned world champion. His promising early days at Lotus earned him a reputation as a brilliant but doomed driver from the start, and he became the butt of pit lane jokes. When Mansell had signed for Williams for the 1985 season, McLaren's Ron Dennis marked the move by screening a compilation video of Mansell's blunders at the 1984 end-of-season F1 party.
Mansell almost provided the perfect riposte by nearly winning the 1986 championship - but incredibly he suffered a blow out with 19 laps left of the season while on course to collect the third place which would have guaranteed him the title.
A year later he missed out again, thanks to a loose wheel nut that cost him a win at the Hungarian Grand Prix, a missed gear change at the Italian Grand Prix that also cost him a win, and a qualifying accident in Japan that forced him out of the final two races and let his team-mate Nelson Piquet win the championship.
Years more ups and downs followed until Mansell finally got a reliable, competitive car at Williams in 1992 which he drove to the championship with a then-record nine wins.
Brilliantly, though slightly shockingly to most spectators, he walked away from the sport at that point. Yet he went with his head held high, and went on to prove his pedigree by winning the Indycar series in the USA the year after.
There's a small postscript to the story: Mansell returned, briefly, to F1 in 1994 to race for Williams following the tragic death of Ayrton Senna, winning his final Grand Prix in Adelaide at the end of the season for the team who he had raced for throughout most of his career.
Sadly, that memory was slightly tarnished with an even briefer dalliance the year after for McLaren, driving without distinction in a car - dubbed, unforgettably, the 'Fat McLaren' - that had to be specially altered due to his by-then increased girth.Bobby Jones quits golf after winning Grand Slam
The Atlanta-born golfer was already a golfing legend before the 1930 season began, having won three US Opens, two Open Championship and four US amateur titles over the previous seven seasons.
What he did in 1930, however, remains unequalled in the sport. He started off in May by winning the British Amateur title at St Andrews, followed up by the Open Championship the month after at Hoylake, added the US Open at Interlachen in July, then completed the Grand Slam - or the 'impregnable quadrilateral', as it was dubbed at the time - by taking the US Amateur title at Merion in September.
Though known as a hot-headed club thrower in his younger days, Jones had matured into one of the most polite and humble champions the game had ever known.
But don't mistake humility for a lack of confidence: Jones was so sure that he would pull off the feat that he backed himself to pull it off with a British bookie at 50-1 before the season started, collecting $60,000 (about £400,000 in modern terms) before walking off into the sunset to concentrate on his career as a lawyer.Donald Bradman denied 100 average with final innings duck
The Australian cricketer, generally regarded as the greatest batsman of all time, walked out to the middle in the first innings of his final Test match looking certain to end his career with a batting average of over 100. Indeed, as he took to the field his average was just over 101.
A seemingly endless ovation reverberated around the ground for the great man, and even the England side who were Bradman's opponents that day gave a spontaneous "Three cheers for the Don!" as he approached the wicket, led by England captain Norman Yardley, who warmly shook Bradman's hand.
Moments later, England's Eric Hollies had soured Bradman's final moment for good. His first ball from the Vauxhall end at the Oval was pushed gently to a silly mid-off; his second was a googly that clean bowled Bradman for a duck.
The BBC's commentator John Arlott wondered if Bradman even saw the ball through the tears in his eyes, for he never got near it, was miles down the crease and would likely have been stumped even if it had missed.
Bradman was clapped off the field, and with 6,996 runs in 70 Test innings his average had dipped to 99.94.
Yet all was not lost: there was still the Australians' second innings to come, and given that the Oval had been one of Bradman's happiest hunting grounds over the years he had ever chance of scoring making the runs he needed to push his average back up over the magic mark.
There should have been, but England had one more bitter part to play in the story: the hosts' second innings batting collapse handed Australia an innings victory, denying Bradman the chance to bat again and score the necessary runs he needed.Rocky Marciano gets off canvas to win final fight
The Italian-American boxing legend is the only heavyweight world champion ever to go through his entire career undefeated, with 49 victories and 43 knockouts in a stellar career.
He took the title in 1952 with a stunning win against Jersey Joe Walcott. After being knocked down in the opening round Marciano was on the ropes throughout, with Walcott building an unsurmountable points lead by the 13th round.
But the Massachusetts-born star landing his trademark "Suzie Q" on Walcott, sending him flopping to the canvas where he remained long after the referee had finish counting 10.
Marciano had one or two other close calls: he had only beat Roland La Starza on a split decision in 1950, and he was taken 15 rounds in a defence against former champion Ezzard Charles, though he went on to beat both fighters convincingly in rematches.
And when he turned up to face ageing Archie Moore, a former light heavyweight champion, just a few days after his 32nd birthday, he had nothing left to prove.
Yet the fight almost went badly wrong in just the second round: Moore floored Marciano with a perfect shot, but the champion managed to beat the count. He then unleashed hell on Moore, knocking him down five times before putting him down for good in the ninth round.
Marciano retired a few months later while still on top of the world, and became a popular boxing commentator and wrestling referee until his untimely death in a plane crash in 1969, aged just 45.
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