Pakistan held their nerve to snap Australia's 34-match unbeaten streak in the World Cup and claim top spot in Group A with a four-wicket success.
Umar Gul claimed 3-30 and Abdul Razzaq 2-8 as three-time defending champions Australia were bowled out for 176 in 46.4 overs on a two-paced pitch at the RPS in Colombo.
Pakistan's run chase was a tense affair, Brett Lee striking twice with the new ball before Asad Shafiq (46) and Younus Khan (31) put on 53 for the third wicket.
But a wobble saw Pakistan pegged back from 98-2 to 142-6, Lee bagging two more scalps to finish with 4-28, before Umar Akmal (44no) and Abdul Razzaq (20no) led them across the winning line with nine overs to spare.
The defeat was Australia's first in the 50-over showpiece since Pakistan beat them at Headingley in the group stage of the 1999 edition.
Having won the toss, Australia lost Shane Watson early on to Gul but recovered well thanks to Brad Haddin and skipper Ricky Ponting, who carefully took the total to 75.
The second-wicket pair's approach was defensive at best but after a 63-run stand, Ponting's poor form continued as he was caught behind off Mohammad Hafeez for just 19.
Haddin kept calm to prod his way to 42 off 80 deliveries before falling to Wahab Riaz.
Hafeez and Shahid Afridi held Michael Clarke and Cameron White in check with some tight spin and the pressure finally paid dividends with White's run-out.
Australia sank further with the losses of set batsmen Clarke (34) and Michael Hussey (12), quickly followed by Mitchell Johnson for a duck.
Jason Krejza (7) became Gul's second victim while Steven Smith's 25 off 32 balls raised some hope before he was bowled by Afridi, as Gul fittingly finished the innings by dispatching Lee for five.
Pakistan's promising start was rocked twice by Lee, who removed openers Hafeez (5) and Kamran Akmal (23) inside the mandatory power play.
Shafiq held the fort thereafter as he and Younus patiently rode out Australia's momentum on the way to a half-century stand for the third wicket.
However, Lee's return to the attack brought fresh joy for Australia as Younus edged straight to keeper Haddin after his patient 31 while Misbah-ul-Haq departed next ball in similar fashion.
Umar's risky hitting in a 41-run stand appeared to have eased Pakistan's nerves but Shafiq was stranded short of his 50, caught in the slips off Johnson just minutes before Afridi's dismissal.
In the end however, with just 34 required off 17 overs, Umar and Razzaq calmly saw their side home, with the latter hitting the winning boundary from the final delivery of the 41st over.