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Blues halfback Ryan Powell and try-scoring winger Lee Abdul both left the field with suspected broken ankles as Mike Ruddock’s Dragons took advantage of having only two players away at the World Cup to record their first away victory.
The Dragons kicked off the scoring with a third-minute try to fullback Sione Tuipulotu after an audacious dummy from scrum-half Gareth Baber, and fly-half Craig Warlow made up for missing the conversion by slotting a 13th-minute penalty goal.
Opposite number Nicky Robinson replied with an excellent 45-metre goal before referee Gregg Davies ignored a knock-on in the Cardiff midfield to give Abdul a free run to the try-line.
Dragons centre Hal Luscombe joined a forward drive to score on the stroke of halftime to give the visitors a 13-10 lead at the break.
Tuipulotu scored his second try early in the second half as both Powell and Abdul were carried off, but a 67th-minute penalty goal from Robinson gave the home fans hope.
Dragons flanker Jason Forster ended that with a pushover try from a close-range lineout – his fifth five-pointer of the season and a bonus point for the visitors.