Scott Cousins went to college at the University of San Francisco. He lives in the Bay Area, and he had almost a dozen friends and family at AT&T Park on Wednesday night to witness the biggest moment of his professional career.

"I don't know if I'm going to be welcomed home as much any more," he said.

Cousins scored the go-ahead run in a brutal collision with catcher Buster Posey at home in the 12th inning, injuring the Giants star as the Florida Marlins beat San Francisco 7-6.

The Marlins blew a four-run lead in the ninth, setting up the play at the plate. Posey went for X-rays on his left ankle, and the Giants said there would be no test results until Thursday, when he'll have an MRI exam.

Emilio Bonifacio hit a shallow fly ball to right-center off Guillermo Mota (2-1) for the second out. Cousins tagged from third base on the sacrifice fly, beating the throw from Nate Schierholtz and lowering his shoulder to slam into Posey for a clean -- albeit cringing -- hit on the reigning NL Rookie of the Year.

Posey's mask went flying in the collision to his chest. Cousins was safe as Posey never could quite corral the ball.

"I felt like he was blocking the dish. It's the go-ahead run to win the game, I got to do whatever I can to score," Cousins said. "I'm not trying to end anybody's season or anything like that. I just was trying to play hard and score the go-ahead run. He didn't say much and you could tell he was in pain.

"And when their manager, when Bruce (Bochy) came out, he was pretty frustrated. I didn't want to make things any more tense."

Posey lay dazed, writhing in pain and curling up in a ball for several minutes as the ballpark fell silent. After several minutes, with fans chanting "Posey! Posey!" he was helped off the field by two team trainers holding his left leg and looking stunned.

There was no immediate update on the severity of Posey's injury. Bochy, a former catcher, didn't believe the play was dirty but said it was difficult to watch.

"It's the toughest play in baseball. You hate to see it," Bochy said. "As a catcher you know what it's like, and you don't like it. Believe me. When I see him laying there, it's certainly not a good feeling."

Said Marlins manager Edwin Rodriguez: "It was a tough play at home. Posey was doing his job blocking the plate and the runner doing his job, he was trying to get to home plate."

All this after the Giants batted around in the ninth to score four runs and force extra innings. Ryan Webb (1-3) pitched a scoreless inning for the win.

This was perhaps the wildest night in a young season already full of them for the defending World Series champion Giants.

Marlins reliever Edward Mujica gave up a single to Schierholtz and a double to Miguel Tejada in the ninth. Leo Nunez entered with one out, and Schierholtz scored on a passed ball by catcher John Buck.

Pinch-hitter Pat Burrell had a bloop RBI single to center, trimming Florida's lead to 6-4. Nunez struck out Andres Torres before Freddy Sanchez doubled to put the tying runs in scoring position.

Aubrey Huff followed with a two-run single to make it 6-all, handing Nunez his first blown save in 19 chances. A good number of fans had left when Mike Stanton's bases-clearing double in the ninth gave the Marlins a four-run lead, and the ones who stayed received quite a show.

Stanton appeared to give Florida all the cushion it would need with a bases-loaded, two-out double to left-center off Javier Lopez to put the Marlins ahead 6-2. Those runs at least gave Florida another opportunity in extra innings after Nunez blew the save, spoiling a sharp outing by starter Chris Volstad.

But the night would belong to a play that could have serious implications for San Francisco's season.

Posey had already taken several foul tips off his mask and leg this season, and some observers have long argued a slugger of his caliber shouldn't be behind the plate -- where hits can be a routine. Posey had even talked about trying a more conventional mask next spring training instead of the hockey-style one he uses.
 
But the mask had nothing to do with this blow.

Cousins said he felt for Posey and repeatedly mentioned that he wasn't trying to injury him. He said he would find a way to send condolences to the other clubhouse and wish Posey well.

"It's a baseball play. I feel bad for Buster Posey, I really do," Cousins said. "I'm going to send a message over there to them."