The Arsenal man scored one of the goals to help Germany on their way to a 4-0 lead with 30 minutes left to play, before the Swedes completed a remarkable comeback to earn a 4-4 draw in their Group C meeting.
The result spells the end of the Germans' 100% start to their World Cup qualifying campaign, and Mertesacker is struggling to come to terms with the events that unfolded in Berlin.
"I really cannot explain what happened here tonight," said Mertesacker. "We are all just very disappointed after a game like that.
"But what I can say is, if you are four goals in front and dominate the match then can't just play with half power. That doesn't work on this level.
"Against a team like Sweden every little mistake gets punished and that is lesson we have to learn."
Mertesacker will be in action for Arsenal on Saturday when they travel to Norwich City.