PITTSBURGH -- Like something from a movie script, the St. Louis Cardinals pulled out a never-say-die, 9-7 win Tuesday over the reeling Pittsburgh Pirates at PNC Park.

The Cardinals had lost an early five-run lead, trailed by a run and were down to their final strike when pinch-hitter Matt Carpenter slugged a tying home run in the ninth off of Pirates closer Tony Watson (2-4) to right-center.

"You're facing one of the better relievers in the game, lefty on lefty matchup. He got ahead 0-2," said Carpenter, whose blast established a major league record with St. Louis' 15th pinch-hit homer of the season.

"I told myself that I wanted it to be short and just battle in that scenario, and got a pitch that I was able to get on top of and hit it out."

Home runs have been contagious for St. Louis, and the effect carried through after Carpenter tied the game at 6-6.

After Yadier Molina doubled, Randel Grichuk hit a two-run shot to left, and Jhonny Peralta followed with a homer to right to make it 9-6.

Mike Mayers (1-1), just called up from Triple-A, pitched a 1-2-3 eighth for the win. Seung-Hwan Oh gave up Jung Ho Kang's second homer of the night in the ninth but picked up his 16th save.

The Cardinals opened a 5 1/2 game lead on the Pirates in the chase for a National League wild-card spot. Pittsburgh has lost eight straight.

"It's tough," Watson said. "A strike away from winning this thing and we're not playing good ball, and to go out there and guys fight back and be a strike away and serve up four more runs, it's tough to swallow. Just go out tomorrow and hopefully get the job done."

Pirates manager Clint Hurdle felt for Watson.

"The life of a late-inning reliever, closer, that's hard duty," Hurdle said. "When you miss like that late in a game, it gets everybody's attention."

So does a comeback like the Cardinals executed after losing a substantial early lead.

"We never quit, that's for sure," said Molina, whose grand slam helped build the early lead.

"I'm speechless," manager Mike Matheny said. "Just, I love how this team competes, right down to the last out."

Both starters lasted just four innings. St. Louis' Luke Weaver allowed two runs on six hits, with five strikeouts and no walks. The Pirates' Ryan Vogelsong allowed five runs on six hits.

Molina hit a grand slam and Matt Adams knocked a solo homer that bounced into the Allegheny River as St. Louis built a 5-0 lead, only to have the Pirates bounce back, thanks heavily to a four-run fifth -- all against reliever Matt Bowman -- that produced a 6-5 Pittsburgh lead.

Molina's fourth career grand slam and first since 2012 landed in the right field stands for a 4-0 lead in the first. Kolten Wong opened the game with a triple to left, Brandon Moss walked an out later and Adams reached on a fielder's choice when first baseman John Jaso hesitated with the ball to look Wong back to third, and everyone was safe.

Molina's slam extended St. Louis' franchise record streak to 25 games with a home run, tying the National League record set by San Diego earlier this season.

Adams' homer in the third was his 14th, a one-out, 436-foot blast over the right field stands to make it 5-0. It was his second homer in as many nights.

Jaso lined a single down the right-field line to drive in Jordy Mercer, who led off the third with an infield single, to pull Pittsburgh to within 5-1.

The Pirates closed it to 5-2 in the fourth when Kang, in his first start and second game back from the disabled list, cranked a solo homer to left.

Pittsburgh's fifth featured four runs, four hits, a walk and two St. Louis errors. Gregory Polanco snapped an 0-for-18 streak with an RBI single to right, scoring Josh Harrison to make it 5-3. Kang followed with an RBI single to center to pull Pittsburgh within 5-4.

Mercer's double to the left-field corner drove in two more to give the Pirates a 6-5 lead before St. Louis' late theatrics.

"I've been a part of too many teams, something like that happens, it's hard to rebound," Matheny said. "You have positive thoughts and all the optimism and you talk about fighting all the way through; it's just very rare to see teams consistently be able to do that after they've had the wind taken out of their sails. To have a five-run lead and watch it go away and to be on the road here, there was a whole lot that happened here tonight."

One of those things was Matheny's instinct to give Carpenter, his regular third baseman, a night off, which meant Carpenter was fresh for the ninth inning.

"Today was a good opportunity (Matheny) thought to give me a break," Carpenter said. "I'm thankful that I got a chance to come in and help us win."

NOTES: Pittsburgh C Francisco Cervelli, who left Monday's game with left thumb discomfort, was not in the starting lineup. ... St. Louis recalled RHP Mike Mayers, RHP Sam Tuivailala and OF Jose Martinez from Triple-A Memphis. ... Pittsburgh added RHP Tyler Glasnow, RHP Drew Hutchison, RHP Trevor Williams and INF Pedro Florimon from Triple-A Indianapolis. To make room on their 40-man roster, the Pirates designated LHP Kelvin Marte and RHP Curtis Partch for assignment. ... RHP Adam Wainwright (Cardinals) and RHP Jared Hughes (Pirates) are the clubs' nominees for the Robert Clemente Award for sportsmanship and community involvement. The winner will be announced during the World Series. ... The Cardinals play exhibition games March 30 at Triple-A Memphis and March 31 at Double-A Springfield.