Scotland seemed on course for their first victory in the Welsh capital since 2002 when they took a 21-9 lead early in the second half.

But tries from Leigh Halfpenny and Lee Byrne got the hosts right back in it and when Scotland were reduced to 13 men Stephen Jones' penalty on the stroke of full-time levelled the scores.

But there was still time for Wales to make the most of their two-man advantage and Williams went over under the posts to break Scottish hearts.

Scotland took a shock lead after just eight minutes when flanker John Barclay barged his way through Gareth Cooper and James Hook's powder-puff tackles en-route to the line.

Jones slotted a penalty only for fly-half Dan Parks to immediately restore Scotland's advantage with a well-struck drop goal.

Parks then turned provider for Scotland's second try on 20 minutes. After the ball had been spread wide, Parks - much maligned by some sections of the Scots support - threaded a perfectly-weighted grubber kick through for Max Evans to touch down at the edge of the in-goal area.

The score was given the thumbs up from the Television Match Official but Chris Paterson missed a kick in the Six Nations for the first time in three years with an off-target conversion.

Fly-half Jones again chipped away at the lead with a penalty after hooker Ross Ford was caught offside only for Parks to respond in kind with an immaculate strike after Wales collapsed a scrum to make it 18-6.

Scotland suffered a double blow when Paterson, who had been carrying an injury since the opening minutes of his 100th cap, went off on the half-hour mark with Thom Evans going off with a serious looking injury soon after.

Andy Robinson threw on Max Evans and scrum-half Mike Blair but before the half was out Stephen Jones was on-target with a penalty to make the half-time score 18-9.

Yet within seconds of the restart, Parks put Scotland further ahead after Byrne was penalised for holding on but Robinson would have been rueing his team's inability to make the most of a two-on-one opportunity spurned by a combination of Sean Lamont and Barclay.

Shane Williams demonstrated how to finish those opportunities on 56 minutes as he delayed his pass perfectly for Byrne to go over in the corner.

After that Scotland enjoyed a good period of pressure and it was rewarded by another Parks drop goal to take the visitors ten points ahead.

Wales turned down opportunities to take penalties but could not fashion try-scoring chances - however they were given a massive boost when replacement Scott Lawson was needlessly yellow carded.

From the resulting lineout Wales worked the ball wide for Roberts to crash his way over only to be denied by the referee for interference by Sam Warburton.

But it proved to be a temporary reprieve for Scotland as massive pressure from Wales eventually allowed Halfpenny the space to burn his way down the touchline and then under the posts.

Jones brought Wales level with a penalty after Phil Godman joined Lawson in the bin for taking out Roberts and there was still time for a dramatic twist in the tale.

From kick-off Scotland opted to keep the ball in play which allowed Wales to secure possession and march their way downfield for one of the most dramatic tries in Six Nations history.