Super Bowl champions New England Patriots fell to a shock opening home loss by Kansas City Chiefs in Boston.
Veteran quarterback Tom Brady failed to register a touchdown pass as the Patriots were beaten 42-27.
Chiefs quarterback Alex Smith threw 368 yards and four touchdowns, but it was rookie running back Kareem Hunt who impressed.
After fumbling on his first carry, the 22-year-old registered three touchdowns and ran 239 yards on his debut.
The Patriots, who in February produced the greatest comeback in Super Bowl history to beat the Atlanta Falcons, began the season as a popular pick to defend their crown, which they achieved in the 2004 season.
But 40-year-old Brady threw 267 yards for no touchdowns and made only 16 of his 36 attempts.
“The only guys who can do something about it are back in that locker room. We’ve got to dig a lot deeper than we did tonight,” said Brady.
The Patriots were leading at half-time and only down by six points at the start of the fourth quarter.
But Brady was unable to find any rhythm as his side faded badly and conceded 42 points – the most they have allowed in coach Bill Belichick’s 17-plus seasons.
Smith, who completed 28 of 35 pass attempts with no interceptions, became the first quarterback with 300-plus yards, four touchdowns and no interceptions against Belichick.
It was also the first time the Patriots have lost at the Gillette Stadium when leading at half-time in 82 games.
They now travel to New Orleans to face the Saints, while the Chiefs host the Philadelphia Eagles.