MOSCOW, June 20 (Xinhua) -- Mexico's 1-0 win over Germany is probably the biggest surprise of the 2018 World Cup finals so far, and has left the reigning world champions with plenty of work to do if they want to qualify for the tournament's knockout stage.

The defeat came following a disappointing set of results in warm-up games leading up to the World Cup, with a draw at home to Spain, defeats against Brazil and Austria and a narrow 2-1 win against Saudi Arabia just 9 days before their loss against Mexico.

Speaking to the press on Wednesday, Germany forward Thomas Muller said the team had perhaps been guilty of not taking those results seriously enough, expecting everything to fall into place once the competition got under way in Russia.

"I think we took things a bit too lightly after the warm-up games," admitted Muller. "Inside the squad, we were self-critical and said we needed to do things, but maybe we thought that in a non-training camp context when the World Cup came around we could raise our game. And we haven't been able to."

Germany need to beat Sweden in Sochi on Saturday, otherwise there is a very real chance their future in Russia would no longer be in their own hands, as a point for Mexico against South Korea would mean that a draw in the final group game between Mexico and Sweden would be enough to see both teams into the last 16.

"What is important now is that we look ahead to the two major tasks lying ahead of us in this group. The pressure is on us and we have to be prepared to go the extra mile," commented Muller.