Mexico overcame having two goals controversially ruled out to beat Group A rivals Cameroon courtesy of a strike from forward Oribe Peralta.
Giovani Dos Santos had a volley and a header disallowed, with replays suggesting both calls were incorrect.
Cameroon rarely threatened but a Samuel Eto’o shot hit the post and Benoit Assou-Ekotto’s free-kick went wide.
However, Mexico secured victory when Dos Santos’s shot was parried before Peralta arrived to slot in the rebound.
At a rain-lashed Estadio das Dunas in north-eastern city Natal, Cameroon almost stole a point late on when Benjamin Moukandjo’s free header was acrobatically saved by keeper Guillermo Ochoa.
An equaliser would have been harsh on a Mexico side who put on an impressive performance, combining a strong work ethic with an attacking threat, and should serve them well when they play hosts Brazil on Tuesday, 17 June.
Peralta’s seventh goal in nine competitive games prevented the dubious refereeing decisions from overshadowing the victory, while Cameroon’s quality and prospects of getting into the second round appear limited with Croatia and Brazil to come.
There were questions about how these countries would fare, with both sides changing managers in scraping through World Cup qualifying.
A nimble Mexico, though, appeared determined to make the most of their time in Brazil, while a powerful-looking Cameroon showed their struggles had continued into the tournament.
The African side struggled to deal with the wing-backs in the 5-3-2 formation employed by the Aztecs, who appeared to have a legitimate early goal disallowed.