Rivers capped an 80-yard drive with an 18-yard touchdown pass to Vincent Jackson with 21 seconds to play, and the San Diego Chargers stunned the Giants, 21-20, yesterday, 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 starting the year 5-0.

“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 [2009] playoffs. The link between me and Eli is always going to be there. So yeah, it’s a little special.’’

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 Co. 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 the Giants settled for Lawrence Tynes’s 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.’’

Rivers was 6 of 8 on the winning march, hitting Malcom Floyd for 12 yards, and later 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.

“Vincent just came open and separated himself from the defender,’’ Rivers said.

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.’’

Rivers also hit Kris Wilson on a 2-yard TD pass in the third quarter to give San Diego a 14-7 lead in a game in which the Giants defense rebounded from 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 a 38-yard field goal early in the fourth quarter.

After the Giants forced San Diego to punt from its 4, Domenik Hixon returned the ball to the Chargers’ 39 and Manning needed only six plays to put New York on top, hitting Kevin Boss with an 8-yard TD strike on a play when the Chargers were offside.

“We played hard, did a lot of good things, but obviously we didn’t do enough,’’ said Giants coach Tom Coughlin.