Greece boss Fernando Santos insists his side still have plenty of work to do, despite placing one foot in the World Cup finals with a 3-1 win over Romania on Friday.
Two goals from Kostantinos Mitroglou either side of a Dimitris Salpingidis strike ensured that the Euro 2004 champions are in the ascendency heading into the second leg, despite Bogdan Stancu's away goal.
The Portuguese coach is pleased that his charges were able to make the most of their chances to score, but warned them not to think that a place in Brazil is assured.
"Nothing is decided yet," the 59-year-old warned to gathered media after the game. "We need to keep a cool head.
"Everything will be decided Tuesday. We have the advantage after the win, but we are only halfway through.
"The players have worked hard and took advantage of the opportunities we had. We hope to do the same during Tuesday's game.
"In the coming days we will be able to release some pressure. Beyond that, we will prepare properly for the next game."
In the other World Cup play-offs on Friday, France were beaten 2-0 in Ukraine, Iceland held Croatia to a home 0-0 draw, and Portugal left it late to beat Sweden 1-0 at Estadio da Luz.