Australia set 254 for victory and a berth in Champions Trophy semi-finals.
Mahela Jayawardene and Lahiru Thirimanne hit aggressive half-centuries as Sri Lanka scored 253-8 in the must-win Champions Trophy match against Australia on Monday.
Jayawardene, who became the eighth batsman to complete 11,000 one-day runs when he reached 62, anchored the middle-order with an unbeaten 84 off 81 balls.
Thirimanne made 57 in the day-night match at the Oval, which Sri Lanka must win to join hosts England in the semi-finals from group A.
Australia need to win in 29.1 overs to make the last four on the basis of a superior run-rate than New Zealand’s.
A loss for either side will knock them out of the eight-nation tournament.
India and South Africa are the semi-finalists from group B.
Sri Lanka make a disastrous start, losing two quick wickets by the fourth over after Australia won the toss in overcast conditions and elected to field.
Left-hander Kusal Perera perished off the third ball of the match, leg-before to left-arm fast bowler Mitchell Johnson, who finished with three wickets for 48 runs.
Kumar Sangakkara, who hit a match-winning 134 not out against England at the same venue last week, made three when he cut Clint McKay to Glenn Maxwell at point to make it 20-2.
Tillakaratne Dilshan (34) helped Thirimanne resurrect the innings with a 72-run stand for the third wicket before the opener was snapped up smartly in the slips by Shane Watson off spinner Xavier Doherty.
Thirimanne hit four boundaries in his half-century when he became Johnson’s second victim, caught at mid-wicket by the alert Watson.
Skipper Angelo Mathews was bowled by James Faulkner for 12 to reduce Sri Lanka to 159-5 in the 37th over before Dinesh Chandimal (31) put on 65 for the sixth wicket with Jayawardene.
K. Perera lbw b Johnson 4
T. Dilshan c Watson b Doherty 34
K. Sangakkara c Maxwell b McKay 3
L. Thirimanne c Watson b Johnson 57
M. Jayawardene not out 84
A. Mathews b Faulkner 12
D. Chandimal c Hughes b Johnson 31
N. Kulasekara run out 6
R. Herath run out 2
L. Malinga not out 2
Extras: (b5, lb7, w4, nb2) 18
Total (for eight wickets, 50 overs) 253
Fall of wickets: 1-8 (Perera), 2-20 (Sangakkara), 3-92 (Dilshan), 4-128 (Thirimanne), 5-159 (Mathews), 6-224 (Chandimal), 7-233 (Kulasekara), 8-244 (Herath).
Bowling: Johnson 10-0-48-3 (nb1), McKay 10-1-51-1 (w3), Faulkner 9-0-60-1 (nb1, w1), Watson 4-0-14-0, Doherty 10-1-30-1, Marsh 2-0-12-0, Maxwell 5-0-26-0.