DENVER -- Buster Posey hit a pair of three-run homers, including a tiebreaking drive as part of a six-run eighth inning, and the San Francisco Giants rallied for a 10-5 win over the Colorado Rockies on Saturday after their bullpen blew a lead.
The Rockies were limited to one run over six innings by Madison Bumgarner, but erupted in the seventh against five San Francisco relievers to take a 5-4 advantage.
San Francisco responded right back with six runs in the eighth. Posey led the charge with a towering homer to right-center off Carlos Estevez (1-2).
Posey also hit a three-run homer in the first inning. It was the third multihomer gamer of Posey's career, two of which have now come against the Rockies. His six RBIs tied a career best.
Cory Gearrin (1-0) threw 1 1-3 scoreless innings to earn his first win since May 28, 2013.
The two teams used a combined 11 relievers.
Bumgarner was at his best in the sixth when he escaped a bases loaded, no-out jam by striking out Ryan Raburn and getting Cristhian Adames to hit into a double play to keep a 4-1 lead in tact.
On his way into the dugout, Bumgarner slapped gloves with teammates. The bullpen couldn't hold the lead.
Nolan Arenado had a two-out, two-run double in the seventh and Carlos Gonzalez followed with a towering, two-run homer to center that gave the Rockies a 5-4 lead and sent the more than 39,000 fans in attendance into a frenzy.
That was soon silenced as the Rockies bullpen couldn't make the lead hold up, either.
Estevez gave up singles to Joe Panik and Matt Duffy before Posey delivered a three-run shot. Jarrett Parker chipped in an RBI single and Gregor Blanco later added a two-run single.
In all, the Giants sent 10 batters to the plate in the pivotal inning and had eight hits.
San Francisco saw Hunter Pence (strained hamstring) and Brandon Belt (sprained left ankle) return to the lineup. Belt had three hits, including wo doubles, and Pence finished with two singles.
Giants: OF Denard Span was a late scratch with a sore right hip. ... The Giants placed RHP Matt Cain on the 15-day disabled list with a strained right hamstring. To take Cain's place on the roster, the Giants recalled RHP Chris Stratton from Triple-A Sacramento. ... RHP Sergio Romo (strained flexor) threw a scoreless inning Friday with Triple-A Sacramento.
Rockies: 2B DJ LeMahieu received the day off. Manager Walt Weiss plans to give some of his regulars a breather over the next week.
Rockies reliever Adam Ottavino threw about "120 mph" in a live batting practice session Saturday as he recovers from Tommy John surgery. At least, that was the velocity according to longtime Rockies great Vinny Castilla, who stepped in to face the Ottavino.
"He was nasty," said the 48-year-old Castilla, who hadn't seen major league pitching since 2006 and managed two foul balls off Ottavino. "He looks great."
Ottavino said he was "letting it go" in the 21-pitch session.
"I didn't sleep very well last night. I was thinking about it. I was excited," Ottavino said.
He will throw another few batting practice sessions before heading out for a rehab assignment.
Giants: RHP Johnny Cueto (7-1) hasn't allowed a run in the last 18 1-3 innings against the Rockies - a streak he will try to extend Sunday.
Rockies: LHP Chris Rusin (1-2) will start Sunday as he takes the place Jorge De La Rosa in the rotation. De La Rosa was recently sent to the bullpen.