Giffin - has thigh trouble. (Getty Images)

World Cup holders Australia have suffered an injury blow ahead of their quarter-final showdown with Scotland at Suncorp Stadium.

Vice-captain David Giffin pulled up with a thigh strain in the Wallabies' final training session and is now rated doubtful for the Brisbane clash.

As Australia's leading line-out jumper, Giffin, who was carried off during the opening game of the tournament four weeks ago, is a hugely influential member of the hosts' squad and his absence would be a major inconvenience.

Justin Harrison is standing by to take Giffin's place with Dan Vickerman likely to move onto the replacements' bench.