Lee was again in stunning form (Getty Images).

By Richard Gibson, PA Sport, Port Elizabeth

Click here for full scorecard
Click here for post-match reaction
Click here for photo gallery
Click here for fantasy leaderboard
Click here for team sections
Click here for more World Cup news

World Cup holders Australia will defend their trophy in Sunday's final in Johannesburg after another clinical display earned a rain-affected win over Sri Lanka at St George's Park.

As in their previous victories in Port Elizabeth, over England and New Zealand, the Australians flirted with a crisis on a lifeless pitch before prospering.

On both those occasions Ricky Ponting's side lost four wickets inside the first 13 overs.

Today, with one less frontline batsman in the side due to Damien Martyn's fractured index finger, they lost three.

Michael Bevan, recalled in place of Martyn after missing Saturday's five-wicket floodlit success over Kenya, was their protagonist previously but his role was taken by Andrew Symonds, who resuscitated the mid-innings with an unbeaten 91 which married responsibility and brute strength.

Walking to the crease at 51 for three with Australia's 15-match winning streak under scrutiny, Symonds buckled down during a fourth-wicket stand of 93 with Darren Lehmann and later opened the shoulders to get his side to 212 for seven, their highest score in three innings on this ground.

That total was always going to be testing for a Sri Lankan side, who scraped through to the last four with a Super Six win over Zimbabwe on Saturday and have been prone to collapses since arriving in South Africa.

The contest was over when Sri Lanka were reduced to 60 for six, although Kumar Sangakkara, attempting to atone for providing Symonds with a life earlier in the day, stretched things out to the 39th over when the deluge began and resulted in a 48-run Duckworth-Lewis success.

Once again it was Brett Lee who struck the blows for Australia with the ball as he worked up a ferocious pace on the docile track.

Captain Ponting, who won the toss and chose to bat first in gloomy conditions, would not have been able to post three slips and a gully but for Symonds' knock which hauled Australia to a match-winning total.

Just six weeks ago, Symonds - who was qualified to play for England by virtue of his birth in Birmingham but favoured Australia - was a scrutinised choice in the world champions' 15-man squad.

But a majestic 143 in difficult circumstances against Pakistan was critical in developing tournament momentum and made the failures of the VB Series seem an age away.

The Queensland and Kent batsman was reprieved on more than one occasion by Sri Lanka, surviving a big appeal for caught behind off Pulasthi Gunaratne, when on four, and a confident leg before shout in the same over.

A missed stumping by Sangakkara off Sanath Jayasuriya, on 33, provided another life and Symonds took advantage to pass 50 for the third time in this tournament from his 69th delivery.

That arrived from a fifth four, powerfully cut off Jayasuriya, and he went on to nurture important lower-order runs, including a huge six over long-on from Aravinda de Silva's off-spin.

Adam Gilchrist's honesty provided Sri Lanka with an excellent start when he walked despite being given not out by umpire Rudi Koertzen.

Left-hander Gilchrist, who smashed 67 off just 43 balls in the five-wicket win over Kenya at the weekend, began in the same manner but succumbed to an inside-edged sweep which popped up off his pad to Sangakkara on the leg side.

Sri Lanka claimed two further early breakthroughs as Chaminda Vaas became the most successful bowler in a World Cup in the process, surpassing the 20 wickets Shane Warne and Geoff Allott, of New Zealand, took in England in 1999.

Left-arm swinger Vaas held one back to break the record as Ponting checked his drive and hit straight to wide mid-off.

The out-of-sorts Matthew Hayden became victim number 22 when when he tucked to the leg side but failed to clear Hashan Tillekeratne at short midwicket.

Vaas returned to claim a third wicket, Ian Harvey caught behind, profiting from the spinners slowing the run rate - Muttiah Muralitharan's 10 overs costing a miserly 29.

But Andy Bichel, whose previous scores of 34 not out and 64 on this ground bred confidence, helped take 16 off the penultimate over, sent down by Gunaratne and Australia passed 200.

And the scene was set for Lee, who took a hat-trick against Kenya on Saturday night and claimed five wickets in a devastating burst against New Zealand at St George's Park a week ago, to charge in once again.

Having had Marvan Atapattu put down at point by Brad Hogg in the fourth over of the innings, Lee followed up undeterred to knock out off-stump with the next ball, timed at 99.48mph.

New ball partner Glenn McGrath removed Jayasuriya, who flicked Lee into the stands at square leg, when the left-hander was cramped for room by a short ball which he guided to square leg.

Sri Lanka made an aggressive initial response to their chase of 213 with Atapattu, whose century against Zimbabwe at the weekend was instrumental in their last four place, striking three boundaries.

But their reinforced middle order - seamer Dilhara Fernando was left out in favour of Mahela Jayawardene - was sliced through once their opening duo fell.

Tillekeratne and Avishka Gunawardene both prodded edges behind the wicket off Lee, who now has 20 wickets in the tournament.

Then, a brilliant piece of fielding from the enthusiastic Bichel accounted for de Silva, in his last international match.

Continuing his delivery stride to swoop on Sangakkara's prod to the leg-side, the bowler turned and threw down the stumps at the striker's end to run de Silva out by three feet.

Bichel was subsequently mobbed by his jubilant team-mates, who sensed victory in the 17th over as Jayawardene's dismal tournament tally reached 21 in seven innings when he fell to Hogg.