England started well on Saturday by bowling Australia out for 385 with the temperature already in the low 40s at the WACA Ground.
By stumps the momentum had swung back to the Australians as the tourists reached 4-180 on a fast and bouncy pitch.
Ian Bell is nine not out and second-gamer Ben Stokes is on 14.
Pietersen was out for 19 in 95 minutes, falling to Siddle for the third time in the series.
The 102-Test veteran Pietersen - the biggest ego in the game - is averaging just 24.00 in the series compared to a career mark of 47.66.
Skipper Alastair Cook's wicket for 72 was another key victory for Australia, while Joe Root (four) left the ground shaking his head at his controversial dismissal after an unsuccessful review.
Cook and Michael Carberry (43) shared a partnership of 85 which is the highest opening stand for either side in the series.
Carberry played a short ball onto his stumps from the bowling of paceman Ryan Harris.
Root looked shocked after he was given out caught behind to seamer Shane Watson.
Inconclusive evidence on Real-Time Snicko and Hot Spot meant Root's fate was in the hands of South African umpire Marais Erasmus, whose original decision stood.
England fans booed as Root made a slow and reluctant exit.
Cook was set for a century in his 100th Test but fell caught at point to spinner Nathan Lyon at 3-136.
Ten runs later came the turning point of the day when Pietersen couldn't contain his aggression any longer. Pietersen swatted a ball to mid-on where Mitchell Johnson took a high-leaping catch.
"It was a crude slap across the line at the most critical time of this Ashes series," ABC commentator Kerry O'Keeffe said.
Cook's men dominated the opening session, taking 4-59 to bowl Australia out for 385.
Steve Smith (111) and David Warner (60) were the only two batsmen in Australia's top six to reach 25. The home side had been 5-143 before Brad Haddin (55) shared a 124-run stand with Smith.
Johnson (39) did not add to his overnight score and Smith followed in the fifth over of the day.
England trail two-nil in the five-match series and defeat in Perth would mean Australia regain the Ashes.
AUSTRALIA FIRST INNINGS
C ROGERS run out (Anderson) 11 (9)
D WARNER c Carberry b Swann 60 (77)
S WATSON c Swann b Broad 18 (29)
M CLARKE c Cook b Swann 24 (44)
S SMITH c Prior b Anderson 111 (208)
G BAILEY c Pietersen b Broad 7 (12)
B HADDIN c Anderson b Stokes 55 (100)
M JOHNSON c Prior b Broad 39 (62)
P SIDDLE c Prior b Bresnan 21 (36)
R HARRIS c Root b Anderson 12 (19)
N LYON not out 17 (25)
Sundries (6lb 3w 1nb) 10
Fall: 13 (Rogers), 52 (Watson), 106 (Clarke), 129 (Warner), 143 (Bailey), 267 (Haddin), 326 (Johnson), 338 (Smith), 354 (Harris), 385 (Siddle).
Bowling: J Anderson 23-5-60-2 (1w), S Broad 22-2-100-3 (1w), T Bresnan 23.3-4-81-1, B Stokes 17-3-63-1 (1w 1nb), G Swann 17-0-71-2, J Root 1-0-4-0.
Batting time: 477 mins. Overs: 103.3.
ENGLAND FIRST INNINGS
A COOK c Warner b Lyon 72 (153)
M CARBERRY b Harris 43 (76)
J ROOT c Haddin b Watson 4 (16)
K PIETERSEN c Johnson b Siddle 19 (59)
I BELL not out 9 (62)
B STOKES not out 14 (43)
Sundries (10b 3lb 5w 1nb) 19
Four wickets for 180
Fall: 85 (Carberry), 90 (Root), 136 (Cook), 146 (Pietersen).
Bowling: R Harris 15-7-26-1, M Johnson 15-6-43-0 (1nb), S Watson 9-2-32-1, P Siddle 13-5-27-1 (1w/5), N Lyon 16-6-39-1.
Batting time: 293 mins. Overs: 68.
Umpires: Billy Bowden (NZL), Marais Erasmus (RSA).