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SAN FRANCISCO (Reuters) - The New Orleans Saints maintained their winning start to the season when Garrett Hartley kicked a last second 37-yard field goal to lift the defending Super Bowl champions over the San Francisco 49ers 25-22 Monday.

The Saints held an eight-point lead but a seven-yard rushing touchdown by Frank Gore capped an impressive eight-play, 82-yard drive before a two-point conversion brought the 49ers to a 22-22 tie with one minute 19 seconds left.

The conversion was initially ruled out but with the referee adjudging that tight-end Vernon Davis caught the pass from quarterback Alex Smith after leaving the end zone.

That on-field decision was over-ruled after a video review however, raising the prospect of over-time if the 49ers could keep out the Saints.

But New Orleans, who were handed four turnovers in the game from San Francisco, marched 51 yards to set up Hartley's game winner.

A false start led to a five-yard penalty, adding to the pressure on Hartley, who was forced to kick in windy conditions. But Hartley, whose overtime field goal secured the Saints their first-ever Super Bowl appearance a year ago, nailed the kick.

It was not a vintage display from the Saints, who beat the Minnesota Vikings in week one, nor was it an entirely discouraging evening from the 49ers who now stand at 0-2 but showed plenty of grit.

BAD START

Saints running back Reggie Bush than raced over for a six yard touchdown before the 49ers fought back with touchdowns from Gore, on a 12 yard Smith pass, and Anthony Dixon on a two yard run.

"He has done a good job for us. He is a good kicker and I am just proud of the way he responded, it was a big play, the conditions were windy," Payton told reporters.

The major worry for the Saints was a leg injury to Bush which took him out of the game and to hospital for tests.

San Francisco coach Mike Singletary was angry about his team's errors, particular the turnovers.

"We had to try and overcome some of the things we did to ourselves, that were self-inflicted..in this league it is very difficult to win football games when you are laying the football down, when you are giving the football up..we have to learn how to protect the football consistently.

"When we do that then I think we will give ourselves the chance to be a good football team," he said.

(Reporting by Simon Evans; Editing by Steve Ginsburg/Alastair Himmer)