"Without a question, I think... they are more of a team under pressure," he said.
"They have done so well in this competition, they have blown away teams and now they are in a situation where if you lose one game, you're gone."
Australia are unbeaten in nine World Cup games and have not been beaten in 15 successive one-day internationals, a world record.
The last time they lost was against the Sri Lankans in January but Ricky Ponting's team exacted their revenge at the World Cup when beating Whatmore's team by 96 runs in the Super Sixes.
"Sri Lanka have had to fight a bit harder," Whatmore, a former Australia test player, added.
"Still in some judges' eyes Sri Lanka is not a powerhouse in world cricket.
"But we have made the semi-finals of the World Cup at the expense of South Africa, who are meant to have been a really powerful team, New Zealand and England."
