Sydney - The Brumbies will approach Saturday's Australian derby with the Waratahs as if their Super 14 lives depend on it.And with good reason, as both sides try desperately to make up ground on the competition's top four.The Brumbies are in sixth place on the table on 26 points three adrift of the fifth-placed Waratahs.Competition surprise packets the Reds moved into fourth position on 30 points, courtesy of their 19-12 upset over the Bulls.The Brumbies could conceivably drop one of their last four games and still make the play-offs for the first time since their 2004 championship.But outside-centre Adam Ashley-Cooper knows the side can not rely on other results to fall their way."I guess now we can't afford to lose again," Ashley-Cooper said on Sunday."This weekend we're playing for our season against the Waratahs, so it's a very important game."We've got our season on the line so it's going to be a pretty important week for us."The Brumbies have been dealt no favours by their run into the finals.Games against top-four sides the Reds and the Crusaders are separated by a visit to Canberra Stadium by the out-of-contention Highlanders.Very few teams leave Christchurch as winners, with the ladder-leading Crusaders having the best record in Super 14 history.The Brumbies missed an opportunity to leap into the top four in their 23-13 loss to the Hurricanes last Friday.Despite controlling the bulk of the play, an inept Brumbies attack was unable to make the visitors pay dearly."I guess that was the most disappointing thing about the game, was the amount of possession we had in the first half and we just couldn't capitalise," Ashley-Cooper said."Basically execution hurt us."One of the positives to come out of the game was Ashley-Cooper's performance at outside-centre.More accustomed to fullback, the return of Julian Huxley to the custodian role has seen the 26-year-old's utility value used to its full extent.Already used on the wing this season, Ashley-Cooper filled in admirably for the injured Stirling Mortlock in the No13 jersey.With Mortlock unlikely to return from a back injury to face the Waratahs, Ashley-Cooper is relishing a chance to be closer to the action."I certainly enjoy seeing the ball a lot more," he said."I haven't played 13 in a while, so I'm certainly feeling it. It's a different fitness to the back three, it's more contact."The Brumbies and the Waratahs have developed a fierce rivalry over the years, with the NSW side holding an 8-7 advantage after 15 matches."It always is a special game against the Waratahs," Ashley-Cooper said.