Tries in the opening attacks of each half gave the Cardiff Blues a 20-12 victory over Ospreys in a thrilling Welsh derby in front of 16,341, the highest crowd yet for a rugby match at the new Cardiff City Stadium.

With flanker Martyn Williams outstanding on his first appearance 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. 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 left-wing Tom James to outpace the Ospreys' cover, tap the ball forward and win the race to touch down. Ben Blair converted.

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

That man 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 sped past two tacklers to send in scrum-half Gareth Cooper, whose overall performance will have boosted his chances of facing the All Blacks in a fortnight's time.

The Ospreys, who brought on Wales skipper Ryan Jones at half-time, made a further switch, 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 nine sparking a move which ended with Ireland's Tommy Bowe crossing by the flag.

But the Blues' defence held 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.