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EAST RUTHERFORD, N.J. -- Five years after being traded away in a draft-day swap for Eli Manning, Philip Rivers came back and stuck it to the New York Giants.

Rivers capped an 80-yard drive with an 18-yard pass to Vincent Jackson with 21 seconds to play and the San Diego Chargers stunned the Giants 21-20 on Sunday, posting their third straight win while handing New York its fourth straight loss.

San Diego (5-3) is now well positioned for the second half of the season. The Giants (5-4) can only shake their heads in disbelief after blowing their first 5-0 start since 1990.

"It's a big emotional win," said Rivers, who threw for 209 yards and three touchdowns. "I don't need a real reason why, but it ranks right up there with the win at Indy in the playoffs [2009]. The link between me and Eli is always going to be there. So yeah, it's a little special. Any time you play against a team that won the Super Bowl, it's fun. It [the trade] didn't weigh into my mind and my thinking, but I bet it was there."

Rivers had the Giants to thank for giving him the chance to pull the game out.

Giants cornerback Terrell Thomas returned an interception 33 yards to the Chargers 4 with 3:14 to play. If Manning and company had punched the ball in, the game would have been over.

But a first-down holding penalty on Chris Snee pushed New York back 10 yards and it settled for Lawrence Tynes' second field goal, a 22-yarder with 2:07 to play for a 6-point lead.

"We had a chance and you can't leave that team in the game," said Manning, who was 25 of 33 for 215 yards and two touchdowns. "You can't leave that team an opening. They are good and talented and if you have a chance to end it, you have to end it."

Rivers did. He hit 6 of 8 passes in the game-winning march, hitting Malcolm Floyd for 12 yards, two to Antonio Gates and a 21-yarder to Darren Sproles that put the ball at the 18. On the next play, Rivers found Jackson in the right corner of the end zone for the game-winner.

"Vincent just came open and separated himself from the defender," Rivers said. "On that play, he was probably the last option to get the ball by the way it set up. But we had all that field and I just wanted to give him a chance to get to it."

Jackson also caught a 10-yard TD pass in the second quarter.

"He's calm and confident and that just rubs off on everyone else," Jackson said of Rivers. "It didn't matter what the score was or how much time was left. We were going to get the job done."

Appropriately, the game ended with Shawne Merriman sacking Manning. San Diego drafted the linebacker in 2005 with one of the picks the Chargers got from the Giants in the Manning deal.

"We showed a lot of team character to bounce back," Merriman said. "We never want to start off slow, but we've learned when we need to hit our stride."

Rivers also hit Kris Wilson on a 2-yard TD in the third to give San Diego a 14-7 lead in a game in which the Giants defense rebounded from a miserable efforts the previous three weeks.

Manning drove the Giants 60 yards in 10 plays on the next possession and Tynes got New York within 14-10 with his 38-yard field goal early in the fourth quarter.

The Giants defense, which gave up 112 points in the first three games of the losing streak, then helped them take the lead, recording three straight tackles for losses and forcing the Chargers to punt from their 4.

Domenik Hixon returned the ball to the San Diego 39 and Manning needed only six plays to put New York on top, hitting Kevin Boss with an 8-yard TD on a play when the Chargers were offside.

A bizarre play at the end of the Giants' opening drive cost them 3 points. New York lined up for a 39-yard field goal attempt, but Tynes never got the kick off. Feagles took the snap and Tynes said Feagles had both hands on the ball so he could not kick it.

"We are all very, very upset, very disappointed obviously in the loss," Giants coach Tom Coughlin said. "We played hard, did a lot of good things but obviously we didn't do enough."

Rivers gave the Chargers a 7-0 lead on the first play of the second quarter with his first TD pass to Jackson. It capped a 45-yard drive after a short punt by Feagles.

The Giants tied the game on the ensuing series on Manning's 6-yard TD pass to Steve Smith. The pair had kept the nearly 11-minute drive alive with a 19-yard completion on third-and-18 from the Chargers 44.

Game notes

Giants weakside LB Michael Boley and DT Chris Canty returned to the lineup. Boley missed a month after knee surgery. Canty was out since the opening week with a calf injury. ... Chargers starting ILB Kevin Burnett was inactive with an ankle injury. ... The Giants decided to honor the Yankees for their recent 27th world championship and introduced manager Joe Girardi during a break in the game. He was identified on the scoreboard as the team's general manager. Does Brian Cashman know?