Australia captain Aaron Finch led by example as his brisk half-century put the World Cup holders in control of Saturday's match against Sri Lanka.
Finch reached his 50 in 53 balls with seven fours to leave Australia on 83-1 after 18 overs at the Oval.
Together with opening partner David Warner, Finch had carved up the rusty Sri Lanka attack after being put into bat under cloudy skies in south London.
Warner was the first Australian to perish when he was bowled by spinner Dhananjaya de Silva for 26, bringing Usman Khawaja to the crease.
Australia were searching for their fourth win in five matches after beating Afghanistan, the West Indies and Pakistan, with their lone defeat coming against India.
Sri Lanka suffered an early scare when fast bowler Isuru Udana injured his shoulder in a failed attempt to stop Finch hitting a four in the fourth over.
Chasing the ball towards the boundary, Udana slid into the advertising boards around the pitch.
He immediately grabbed at his left shoulder in pain and needed treatment before walking to the dressing room with an ice-pack on the injured area.
Fortunately for Sri Lanka, Udana returned to the field and was able to start bowling soon after.
The rest of his team were struggling to find their rhythm after two successive wash-outs robbed them of game-time.
Three sloppy pieces of fielding in the space of eight balls led to six extra runs for Australia.
Finch and Warner were already well to top without Sri Lanka's mistakes and they reached their 50 partnership in 58 balls.
It was their fourth opening stand of at least 50 in the tournament and their 19th in all ODIs.
Finch survived an lbw replay review when he was on 35 as he progressed serenely towards a big score.
Sri Lanka's previous two matches against Pakistan and Bangladesh had been abandoned due to rain, meaning they were back in action for the first time in 11 days.
That frustrating down-time allowed grievances to fester and team manager Ashantha de Mel had taken a swipe at World Cup organisers earlier in the week for allegedly giving preferential treatment to other countries.
De Mel blasted the quality of pitches, practice facilities, transport and accommodation at the World Cup, prompting a stern defence from the International Cricket Council.
Crushed by New Zealand in their opening game, Sri Lanka had bounced back to beat minnows Afghanistan in another rain-affected clash.
Sri Lanka bowl against Australia in World Cup
Sri Lanka captain Dimuth Karunaratne won the toss and elected bowl in Saturday's World Cup match against holders Australia.
Karunaratne opted to take advantage of cloudy conditions and a moist pitch at the Oval as he looked to put Sri Lanka in position for their second win of the tournament.
"The last few days, it has rained so we want to take advantage of the moisture in the surface by bowling first," said Karunaratne, whose team had seen their previous two matches abandoned due to rain.
Australia are aiming for a fourth win from their five matches and skipper Aaron Finch admitted he would have bowled if he had won the toss.
"I haven't won a toss for such a long time. I would have bowled first too. I think it will swing around so we'll have to bat well," Finch said.
Australia made one change, with Jason Behrendorff replacing Nathan Coulter-Nile who was ruled out after feeling "a little bit sore".
Australia: David Warner, Aaron Finch (capt), Usman Khawaja, Steve Smith, Shaun Marsh, Glenn Maxwell, Alex Carey (wkt), Pat Cummins, Mitchell Starc, Jason Behrendorff, Kane Richardson
Sri Lanka: Dimuth Karunaratne (capt), Kusal Perera (wkt), Lahiru Thirimanne, Kusal Mendis, Angelo Mathews, Milinda Siriwardana, Dhananjaya de Silva, Thisara Perera, Isuru Udana, Nuwan Pradeep, Lasith Malinga
Umpires: Aleem Dar (PAK) and Richard Illingworth (ENG)
TV umpire: Michael Gough (ENG)
Match referee: Jeff Crowe (NZL)
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