A night after Nationals phenom Stephen Strasburg shut down the Giants, Posey hit a tiebreaking single in a four-run seventh inning and added a two-run homer that withstood a video review to lead San Francisco to a 10-5 victory over Washington on Saturday night.
Brought in to protect a 5-4 lead in the seventh, Tyler Clippard (8-6) allowed Aaron Rowand's one-out homer to left on a 3-2 changeup. Freddy Sanchez and Aubrey Huff walked, and both were running when Posey sent a single through the hole between first and second to put the Giants ahead.
"Full count, I was just battling," said Posey, who had his second career four-hit game in four nights. "I knew from watching some previous at-bats, [Clippard] had a lot of confidence in his changeup, and I knew if I was going to get beat, I didn't want to get beat with the changeup."
Juan Uribe then hit a two-run double to right-center for an 8-5 lead.
Clippard has been a dependable setup man, and leads the Nationals' pitching staff in victories, but is 0-3 with 11 runs allowed in his last 6 1/3 innings over seven appearances.
"Tyler's just struggling. ... He's gotten hurt a little bit with his changeup," Washington manager Jim Riggleman said. "He's got a great changeup, but word gets around the league a little bit and every now and then maybe a hitter's looking for it."
In the ninth, Posey hit a drive to right-center off Miguel Batista that was bobbled and caught by a fan in the first row wearing a Giants cap. The fan appeared to reach over the railing, but the umpires used a three-minute review to uphold the call.
"It wasn't a great feeling," Posey said of the brief stoppage. "I really didn't want to go out there to second base, so I was glad it held up."
It was Posey's seventh homer since being recalled from Triple-A Fresno on May 29 -- and his sixth in 10 days. In July, the 23-year-old is hitting .514 and has 13 RBIs.
"I try not to have too many expectations," he said. "I try to set myself a goal, whether it's for a year or a week or a day -- just go out and play as hard as I can and the results take care of themselves."
From behind the plate, Posey also made two key defensive plays in the sixth to short-circuit a Washington rally. He threw out pinch-runner Roger Bernadina at third as he tried to advance on Ian Desmond's attempted sacrifice and later nabbed Desmond trying to steal third.
"Big play. We were in a bit of a jam there and [Posey] made a great throw to third [on the bunt]," Giants manager Bruce Bochy said. "He batted well with runners in motion, he found a hole. And at the end, those were big runs to tack on."
On the one-year anniversary of his no-hitter against San Diego, Giants left-hander Jonathan Sanchez lasted only 3 2/3 innings, yielding five hits and five runs. He walked two and struck out six in his shortest outing of the year.
Santiago Casilla (2-2) got the final two outs of the sixth.
Uribe also had three RBIs for San Francisco, which has won six of eight games.
Sanchez helped himself out with a two-run single off Craig Stammen that gave the Giants a 3-2 lead in the second. With runners on first and second, Nate Schierholtz doubled into the right-field corner, scoring a run, and Sanchez followed with a single to center.
Mike Morse homered to center leading off the Nationals second.
Washington chased Sanchez in a two-run fourth. Josh Willingham legged out an infield single and scored on a double to left by Ivan Rodriguez. With two down, Stammen's ground-rule double made it 5-3 and Sanchez was pulled.
Stammen departed in the sixth after Uribe's sacrifice fly got San Francisco to 5-4. The right-hander yielded four runs and six hits.
Washington ended a franchise record streak by not allowing a first-inning run for the first time in 10 games. ... Maxwell was recalled from Triple-A Syracuse before the game. ... Uribe (sprained left ring finger) returned to the Giants lineup after missing four games, but OF Andres Torres (left groin tightness) sat out. ... Nationals C Jesus Flores, who has been on the disabled list all season while rehabilitating a right shoulder injury, has been shut down until after the All-Star break because he is experiencing pain, said general manager Mike Rizzo.