Mexico stunned World Cup holders Germany on Sunday, inflicting a 1-0 defeat that throws the Germans' title defence into disarray as Neymar's Brazil stuttered to a draw against Switzerland.
The outstanding Hirving Lozano smashed home the Mexican winner in the 35th minute at Moscow's Luzhniki Stadium, leaving Manuel Neuer clutching at thin air on his return to the German goal after a nine-month injury absence.
Toni Kroos crashed a shot against the crossbar for Germany but they were never able to get back into the match as they slid to their first defeat in their opening game of a World Cup since 1982.
"I don't know if it's the biggest win in Mexico's history but doubtless it is one of the biggest for sure," said 22-year-old Lozano.
Germany coach Joachim Loew did not mince his words about his side's performance, saying his team had played "very badly" in the first half of the Group F tie.
"We weren't able to impose our usual way of playing, our attacking and passing was not effective," he said.
Kroos said the Germans had no choice but to win their next two group games as the pressure mounts on Loew to turn their fortunes around against Sweden and South Korea.
If Germany finish as group runners-up, they could face Brazil in a clash of World Cup titans in the next round -- a perfect chance for the Brazilians to avenge the humiliating 7-1 defeat to the Germans on home soil in 2014.
Brazil, and their star Neymar, made their bow in Russia in Rostov-on-Don but gutsy Switzerland battled back to cancel out a Philippe Coutinho strike midway through the first half of the Group E tie.
The Barcelona man found the top corner with a stunning curled effort in the 20th minute but Steven Zuber headed an equaliser from a corner shortly after half time and the five-time champions could not make their superior quality count.
Four years after injury cut short his World Cup -- Brazil suffered that defeat to Germany without him -- Neymar is once again spearheading his country's hopes.
The Paris Saint-Germain striker's participation at the finals in Russia had been cast into doubt after he suffered a broken bone in his right foot in late February.
However, the 26-year-old forward has shown few signs of rustiness since returning for Brazil, scoring in consecutive friendlies in the buildup to the finals and started on Sunday.
- German disarray -
Germany had breezed through qualifying, scoring 43 goals and conceding just four.
Loew has, like the Brazilians, transformed the team from 2014 to the extent that the man who scored the World Cup winning goal in Rio, Mario Goetze, is not in the squad.
But his plans to become only the third side in the World Cup's 88-year history to successfully defend their title were in disarray on Sunday after the Mexicans tore into his defence from the start.
Mexico only lost one game in qualifying and have reached at least the second round of every World Cup they have played.
In the first game of the day, Serbia beat Costa Rica 1-0 thanks to a brilliant Aleksandar Kolarov free-kick in Samara.
The victory puts Serbia in a strong early position in Group E ahead of games against Brazil and Switzerland as they look to reach the last 16 for the first time since becoming an independent country.
Away from the football, Russian authorities said a taxi driver who ploughed into pedestrians on a crowded pavement near Red Square in Moscow on Saturday, injuring at least seven people, had been suffering from exhaustion.
The taxi driver, a 28-year-old man from the central Asian republic of Kyrgyzstan, told police during questioning that he had been working for 20 hours solid before the incident and had mixed up the brake pedal and the accelerator.
And a space for gay and ethnic minority football fans in Saint Petersburg during the World Cup was forced to relocate at the last minute in a move activists believe was politically motivated.
Violence against gays in Russia regularly makes global headlines.