* Record test crowd of 91,092 flock to MCG
* Pietersen responds to critics with unbeaten 67
* Watson suffers groin injury (Writes through)
By Ian Ransom
MELBOURNE, Dec 26 (Reuters) - Mitchell Johnson's late double strikes fired up a world record test crowd at the Melbourne Cricket Ground and left England a rocky 226 for six after Kevin Pietersen threatened to win the opening day of the fourth Ashes match for the tourists.
Roared on by a heaving crowd of 91,092, paceman Johnson steamed in to remove number six Ben Stokes (14) and wicketkeeper Jonny Bairstow (10) in quick succession to put Australia marginally ahead at the end of a meandering Thursday's play.
Coming in to the match under a barrage of criticism, Pietersen responded with a defiant half-century after being granted lives on six and 41, and went to stumps unbeaten on 67, with Tim Bresnan hanging on desperately with one run.
With seamer Ryan Harris having Ian Bell caught behind by wicketkeeper Brad Haddin for 27 in the first of three wickets after tea, Australia's bowlers worked hard to validate captain Michael Clarke's surprise decision to field after winning his fourth successive toss of the series.
The hosts could have been in a stronger position, however, had they not let three catching chances go down, with two reprieving Pietersen. Their bowling attack also suffered a blow with all-rounder Shane Watson sustaining a groin injury.
Watson pulled up in his runup and left the ground after lunch, and though he later returned to field in the slips, he did not bowl again.
"It was difficult, I think Michael (Clarke) wanted to lose this (toss)," Harris, who bowled superbly to finish the day 2-32, told reporters.
"We didn't start as well as we liked ... Once again we pulled it back and 6 for 220 the end of the day and with three chances down as well is a pretty good day (for us)."
Although quelling his usual attacking instincts, Pietersen was at his theatrical best in his 152-ball knock, slumping to his knees twice in an apparent bout of nausea after smacking a shot to midwicket where a leaping George Bailey put a tough chance down with the batsman on 41.
The crowd smelled gamesmanship as play was paused for a few minutes and Pietersen later enjoyed riling up the terraces again by holding up Johnson's run-up in the final over.
"It's a great sign to see him scrap it out and hopefully he's one guy in world cricket that if it does click tomorrow morning could be some good cricket if he comes through aggressively," England batsman Bell told reporters of Pietersen.
Pietersen could have been due for another round of criticism when he slogged to deep backward square when on six, but after taking the catch, substitute fielder Nathan Coulter-Nile stepped over the boundary line in a comical reprieve for the South Africa-born batsman.
Pietersen went on to dig in for a defiant 67-run stand with Bell before the lionhearted Harris struck. Harris unleashed a sublime delivery that jagged away off the seam and had Bell feathering an edge after he had become only the second batsman to surpass 1,000 runs in tests for the year with Clarke.
The Australian attack derived little from the drop-in pitch early but restricted England to 135 for three at tea with a disciplined line and length that saw the tourists' run rate slow to crawl.
Before coming off injured, Watson bowled opener Michael Carberry for 38 before a flat-footed Joe Root was caught behind by Haddin off the bowling of Harris for 24.
The catch was the revitalised Haddin's 50th in a year in which he has also scored more than 500 runs with the bat.
Siddle earlier continued England captain Alastair Cook's forgettable series by dismissing him for 27 in the morning, breaking a promising 48-run opening stand with Carberry.
Carberry nicked behind when on two runs, but a diving Steven Smith put down a difficult chance that he might better have left for second slip Clarke.
England, who have already lost the five-test series 3-0 to surrender the coveted urn, dropped wicketkeeper vice captain Matt Prior, who paid the price for a poor campaign with the bat and behind the stumps.
Yorkshire's Bairstow, recalled for the first time since the northern Ashes series, lasted 19 minutes at the crease before being bowled by Johnson. (Editing by Amlan Chakraborty)