Chasing 123 for victory at the WACA Ground, Australia crashed to 3-10 in the fourth over after Alyssa Healy, Beth Mooney and Ashleigh Gardner fell for single figures.

A loss would have all but ended Australia's World Cup defence given they also lost to India last Friday.

But the tournament favourites dug deep to pull off a face-saving win, with Haynes (60 off 47 balls) and Lanning (41no off 44 balls) combining for a 95-run stand.

Haynes, who was given out LBW on 15 before having the decision reversed on review, received a slice of luck on 26 when dropped at mid-on by Umesha Thimashini.

Australia would have fallen to 4-59 if the catch had stuck but the missed opportunity came back to haunt Sri Lanka.

Haynes struck consecutive sixes in the 16th over to well and truly swing the match Australia's way.

It meant they needed just 26 runs off the final four overs and got home with three balls to spare.

"I actually was feeling ok, particularly walking out there," Haynes said of the nerve-wracking early collapse.

"I'm pretty fortunate that one of the best batters in the world is standing at the other end. Meg and I have played a lot of cricket together.

"We knew it was just a matter of calming things down and getting a really good partnership together. I think we were able to consolidate and do that."

Lanning, playing her 100th T20 international, was a rock under the fiercest of pressure, with the skipper combining brilliantly with vice-captain Haynes.

But Sri Lanka would have been cursing their luck after burning an early review, which meant they couldn't review Lanning's edge behind when she was on 15.

'We thought the first one was edged but unfortunately we lost the review. And the second one was out I think," Sri Lanka captain Chamari Atapattu said.

Udeshika Prabodhani's left-arm swing caused Australia all sorts of problems early, with Healy unable to believe her bad luck when the second ball she faced seamed wickedly to knock over the stumps.

In Prabodhani's next over, Gardner also had her castle demolished.

Things looked dire for Australia when Mooney was stumped after missing a viciously spinning delivery from Shashikala Siriwardena.

Their title hopes were hanging by a thread. It was an unthinkable situation.

The home side crawled to 3-19 from their six-over powerplay but Lanning and Haynes steadily increased the run rate to ensure the hosts always remained within striking distance.

Earlier, Sri Lanka opener Atapattu cracked 50 off 38 balls to lift her side to 6-122.

She struck seven fours and two sixes in the quickfire knock, with Anushka Sanjeewani (25) and Umesha Thimashini (20) chipping in with handy scores.

Australian Associated Press