With flanker Martyn Williams outstanding in his first game back after injuring a shoulder on the Lions tour, the Blues fully deserved their triumph but needed stern defence to withstand a late onslaught as the Ospreys brought their big guns off the bench.

The home side could not have asked for a better start. Their Australian fly-half Sam Norton-Knight, whose introduction in the Blues' line-up has not been smooth, showed what he is capable of with an inch-perfect kick for winger Tom James to outpace the Ospreys cover, tap the ball forward and win the race to touch down.

Ben Blair converted to put the men from the Welsh capital 7-0 in front, with only two minutes on the lock.

Ospreys number 10 Dan Biggar had an opportunity to cut the deficit when the lively Shane Williams, captaining the region for the first time, was taken out as he chased a kick ahead, but the youngster was off target with the resulting penalty kick.

The Blues were doing well at the breakdown, thanks largely to the tireless Williams, but with both fly-halves relying too much on high kicks into the windy night, neither side was able to find a way through.

The only other score of the first half came when Blair punished a high tackle on Jamie Roberts, though Blues winger Leigh Halfpenny hit a post with an injury-time penalty from inside his own half.

The evergreen Williams was key to the Blues repeating their explosive opening after the restart.

The red-headed predator found a minute gap on the left touchline and charged past two tacklers to send supporting scrum half Gareth Cooper through for a timely try.

Cooper's overall performance will have boosted his chances of featuring for Wales against New Zealand in a fortnight's time.

The Ospreys, who brought on Wales skipper Ryan Jones at half-time, made a further switch when moving Shane Williams to the scrum half position in which he started his career.

It paid immediate dividends, a little dart by the stand-in number 9 sparking a move which ended with Irish star Tommy Bowe crossing for a well-taken try.

With James Hook also called into action to add his flair in midfield, the Ospreys suddenly looked a lot more creative, and hooker Richard Hibbard stretched over the line only to be called back for crossing.

But some sterling defence from the Blues saw them hold out and they made the game safe when Norton-Knight dropped a neat goal.

There was just time for New Zealander Jerry Collins to crash over in the corner, Biggar converting, but it was not enough to earn the Ospreys a losing bonus point.

Shane Williams, one of the Ospreys' best performers on the night, said afterwards: "It was disappointing to lose but the effort was there from the boys from start to finish.

"They threw everything at us. Our discipline was quite poor and you can't give the Blues the ball because they are going to score points from it.

"We've got to pick ourselves up for the games to come."

After enjoying his first game back from injury, Martyn Williams was just pleased to have played his part in Cardiff's first win over the Ospreys in four meetings and what was only their second league victory since last May.

"A win was the most important thing, we've struggled against the Ospreys for the last couple of years," admitted the 34-year-old Williams.

"The lungs and legs were struggling and I was feeling it towards the end but it was a good one to come back into.

"The shoulder's fine hopefully, I was more worried about just getting through it."

Asked about his probable involvement in Wales' November games, he added: ""Fingers crossed (for the autumn Tests). It's Leinster away next week, I want to play in that and who knows from there?"

Tonight's game continued Cardiff's impressive string of results against Welsh opposition - they have now won 12 of their last 14 home games against their regional rivals.