On a day of upsets in the Under-19 World Cup, Zimbabwe beat Australia and Nepal toppled South Africa.
After a seven-wicket loss, Australia must beat Sri Lanka on Friday to keep their title defence alive.
India strengthened their challenge with a 69-run victory over New Zealand, bowling the Kiwis out for 146 in Dhaka.
England, Sri Lanka and Pakistan - none of whom played on Wednesday - are the other teams to have won their first two matches in the group stages.
Zimbabwe's Tinashe Panyangara claimed 6-31, the second-best bowling figures in the tournament's history, to bowl Australia out for a team record low 73.
In front of a full house of 15,000 in Bogra, Zimbabwe took just 18 overs to topple the total.
It was a remarkable batting collapse after openers Tim Paine (19) and Theo Doropoulos (26) had helped Australia to 50-1 after 10 overs.
But their other eight batsmen managed only seventeen runs between them, Elton Chingumbura picking up the remaining four wickets.
Zimbabwe captain Tinotenda Mawoyo said it was a brilliant feeling to beat the holders.
He said: "Our morale is now very high. We played very well today and we hope to do the same in our next matches."
Australia coach Bennett King paid tribute to a fine Zimbabwe performance.
He said: "Everything about their game was perfect. I can't remember a game where we have been so completely dominated."
Source