The pitch has favoured batsmen over the first two days, but McDermott has already seen signs that is about to change.
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"I think the cracks are going to get bigger, there's no doubt about that," McDermott said after Australia claimed two key England wickets in the final session to leave the tourists battling at 4-180.
"They grew quite a lot today from what I've seen and that will continue. There's a couple in line with the stumps, so that's going to play havoc on batsmen's minds.
"It may never even hit those cracks, but again that's a psychological thing from a bowler's point of view.
"We've just got to play the best we can on that wicket, but it may be better that we're bowling last on it because the cracks will be even wider."
Australia dismissed England captain Alastair Cook and star batsman Kevin Pietersen late on day two to grab the ascendency in the potentially series-defining Test.
But McDermott said Australia still had plenty of work to do before it could think about taking hold of the urn for the first time since 2009.
"There's a fair bit of cricket to go in this Test match and (Ian) Bell's still in and has an unbelievable record," McDermott said.
"(Matt) Prior averages nearly 50 or just on 50, so they've got some good batting to come and we've got to make sure that we start the day as we finished – on the ball and dotting our i's and crossing our t's.
"More importantly, getting that ball up and swinging it and then grabbing the new ball and getting stuck into the tail when and if that happens."
McDermott said Peter Siddle's ability to bowl tight to Pietersen contributed to the England batsman's downfall.
Siddle has now dismissed Pietersen 10 times in Test cricket, including each of his past three innings in this series.
"Kevin was four off 40 balls and he was digging himself a fair hole from my perspective and then he started playing a shot a ball," McDermott said.
"Sids has just tied him down time and time again and then the release valve comes and Sids cracks him open. That's great for us and we hope that continues."