Germany became the third defending champion in the last 16 years to lose its opening match at the World Cup, falling to Mexico 1-0 Sunday.
Hirving Lozano scored the lone goal in the 35th minute, picking up Javier Hernandez's pass inside the penalty area and beating Mesut Ozil before shooting past Manuel Neuer from 10 yards.
"I don't know if it's the biggest victory in (Mexico's) history, but one of the biggest for sure," Lozano said. "It's great to start on the right foot when you are playing the world champions."
France in 2002 and Spain in 2014 were the other defending champions to lose their opening matches. Neither of those teams advanced from the group stage.
After Lozano's goal, Mexico goalkeeper Guillermo Ochoa pulled off a spectacular save to keep the score even, palming Toni Kroos' shot onto the crossbar.
The Germans are bidding to become the first team to retain the World Cup title since Brazil in 1962. They hadn't lost an opening game since the then-West Germany succumbed 2-1 to Algeria in 1982.
Mexico supporters outnumbered Germany fans at the Luzhniki Stadium and made almost all of the noise, roaring their team forward.
Lozano, a 22-year-old forward from the Dutch league nicknamed "Chucky," got Mexico going with a deflected shot over the bar in the first minute. That set the tone for Mexico to torment Germany on the counterattack, with Lozano taking full advantage of right back Joshua Kimmich's tendency to go forward.
In his first competitive start since Sept. 16 because of injury, Neuer was kept busy saving a stinging long shot from Hector Herrera in the 10th, then a header shortly after a free kick.
Timo Werner could have scored when he picked up a ball near the penalty spot, but his shot went straight to Ochoa.
With key players tiring in the second half, Mexico pulled back and substituted both Lozano and Carlos Vela. Germany then dominated but when it broke through the Mexican defense, the shooting was poor.
Seeking defensive reinforcement, Mexico coach Juan Carlos Osorio brought on 39-year-old veteran Rafael Marquez to play in his fifth World Cup.