"Some people may think that it's somewhat of a selection trial, but from our perspective, it's only the third match of the Super 14," Mortlock said.
"It's another match, and it's one that we want to be up for, and it's an opportunity to hopefully play a little bit better in attack.
"The one-on-one battles, selectors may take a look at, but consistency and form over the whole Super 14 is what probably warrants selection at the end of the competition.
"From our perspective, it's very early days and we're solely focused on playing well as a team on Saturday night."
Brumbies coach Laurie Fisher agreed with Mortlock, saying his side would have to concentrate on the game rather than external factors.
"All matches are intense," he said.
"We just have to make sure that the local derby factor doesn't come into the equation.
"It's just about concentrating on your performance and trying to execute it. If we do that, we'll be a good chance at winning."
With the typically tough nature of derby matches, Brumbies lock Adam Wallace-Harrison, who will start the match at flanker, urged his team to continue its disciplined start to the tournament - only 12 penalties conceded over the first two matches.
"We've worked hard on minimising the number of penalties we give away, particularly at the breakdown," Wallace-Harrison said.
"The Reds have strung together two strong performances and we know they'll be tough in Brisbane."
The Reds go into the match as rank underdogs, having won just one from 12 super rugby games over the past decade against the Brumbies, including suffering a 36-0 drubbing ending to last season in Canberra.