The tireless seamer produced an excellent spell of swing bowling to record figures of 5-50 as England were bowled out for just 215 after winning the toss.
James Anderson and Steven Finn each took two wickets after tea to reduce the tourists to 75-4 at the close, but Siddle feels that the changing conditions on Thursday and the batting that they have to come puts Australia in a favourable position.
He told reporters: "It's been an extraordinary day. Once we all settled down in the field, things went our way. Rocking up today, we wanted to bat and to get the job done with the ball was good. Hopefully we can now go on and get some runs.
"I don't think the pitch played a big part in the day's play, it was the cloud cover. Hopefully the sun will come out tomorrow and we can set up the match. If we can get in there, dig in and be patient there is still Brad Haddin to come and a few of us tail-enders can still bat. I think we are still on top."
You can follow live coverage of the second day's play from Trent Bridge with Sports Mole from 10.30am on Thursday.