SAN FRANCISCO -- The last time the San Francisco Giants faced the St. Louis Cardinals at AT&T Park, a 24-year-old Double-A nobody named Kelby Tomlinson had just finished a Fall League contest in Arizona and rushed back to his hotel to catch what turned out to be the final game of the 2014 National League Championship Series.

Almost a year later, Tomlinson was no longer watching. He was living it first-hand.

The rookie second baseman lined a single to center field to score Brandon Belt from third base in walk-off fashion with one out in the bottom of the ninth inning Friday night, giving the Giants a 5-4 victory over the Cardinals in the opener of a three-game series.

"Unbelievable," Tomlinson assured after following up his first career home run -- a grand slam -- one day earlier with his first big-league walk-off. "You couldn't script it."

The Giants' third consecutive win featured a franchise-record eighth grand slam, a third-inning shot by right fielder Marlon Byrd that provided all of San Francisco's scoring before the decisive ninth.

The win allowed the Giants (69-59) to remain 2 1/2 games behind the Los Angeles Dodgers (71-56) atop the National League West.

The St. Louis loss, meanwhile, coupled with Pittsburgh's win over Colorado, allowed the Pirates (78-49) to move within 3 1/2 games of the first-place Cardinals (82-46) in the NL Central.

"We just couldn't get a whole lot going," observed Cardinals manager Mike Matheny. "They worked hard to get back into it. Just one of those we couldn't steal."

The St. Louis-San Francisco matchup was the first at AT&T Park since Game 5 of the 2014 NLCS, when the Giants ended the Cardinals' season on Travis Ishikawa's three-run home run in the last of the ninth.

Tomlinson's game-winner only went about half as far, but it sent the Cardinals along the same dejected path to a losing clubhouse.

"There's a lot of fight in this team," insisted Cardinals right fielder Stephen Piscotty, whose team had entered Friday's contest on a five-game winning streak and had won two of three from the Giants earlier this month in St. Louis. "We came up short tonight, but that's all right. It was impressive that we fought back."

Belt's second hit of the night, a single off Cardinals left-hander Kevin Siegrist (5-1), got the winning uprising going.

Catcher Buster Posey followed with a walk, advancing Belt into scoring position, and both runners moved up another 90 feet when center fielder Gregor Blanco drew Siegrist's second walk of the inning with one out.

Facing a five-man infield, Tomlinson then stroked his two-strike game-winner.

"That's exciting," Byrd said of Tomlinson, whose RBI was his 12th in 14 starts. "Big two days and he came through."

The Giants won despite blowing a 4-0 lead, which was the product of Byrd's eighth career grand slam. It came off Cardinals starter Michael Wacha, against whom Byrd had been 0-for-17 previously in his career.

Wacha also was the guy who surrendered Ishikawa's historic blow last season.

The grand slam was Byrd's first since also connecting at AT&T Park on July 9, 2013, but that time for the Cincinnati Reds against the Giants.

"This one was a lot bigger," he assured. "We're in a pennant race and the Dodgers had already won."

After bouncing back quickly to get within 4-3 with a three-run fourth inning, the Cardinals forged a 4-4 tie in the sixth inning on a wild pitch by Giants starter Mike Leake that scored center fielder Tommy Pham.

Neither starting pitcher got a decision.

Leake, making his home debut, left one out into the seventh inning, having allowed four runs on six hits. He walked one and struck out four.

Giants left-hander Josh Osich inherited a two-on, one-out situation from Leake and retired the only two Cardinals he faced to retain the 4-4 tie.

Right-hander Sergio Romo and lefty Javier Lopez followed Osich with two innings of one-hit relief. Lopez (1-0) got the win.

Wacha, a victim of two Cardinals errors before Byrd's grand slam, did not allow an earned run in his six innings. He gave up four runs on six hits and two walks, striking out six.

Tomlinson, Blanco and Belt had two hits apiece for the Giants, who out-hit the Cardinals 10-7.

Piscotty had two hits and two RBIs for the Cardinals.

Defensive miscues directly led to six of the game's first seven runs.

Byrd's grand slam came on the first pitch after Wacha hit Posey on the elbow with a fastball, loading the bases.

The inning should have ended even before Posey came to the plate. Errors on Cardinals third baseman Matt Carpenter and shortstop Jhonny Peralta sandwiched the first out of the inning, putting two runners aboard and setting the stage for Posey and eventually Byrd.

The Cardinals countered immediately on a one-out RBI single by catcher Yadier Molina in the fourth.

Leake almost got out of the jam when he induced the Cardinals' Kolten Wong into hitting a hard grounder to Tomlinson. But Giants shortstop Ehire Adrianza bobbled the ball after getting a forceout at second, denying the possibility of a double play that would have ended the inning.

Piscotty hit Leake's next pitch for his two-run double, narrowing the Giants' lead to 4-3.

NOTES: The walk-off win was the Giants' fifth of the season. ... The Giants lead the majors in grand slams with eight. ... The grand slam allowed by Cardinals RHP Michael Wacha was the first of his career. ... Cardinals closer RHP Trevor Rosenthal did not make the trip from Phoenix to San Francisco. Instead, he headed home to witness the birth of his second child. He was placed on the paternity list and will miss the entire series against the Giants. ... The Cardinals on Friday named Randy Flores as their new director of amateur scouting. He fills the void created when the club fired Chris Correa after he had been accused of hacking the Houston Astros database earlier this season. ... Cardinals OF Jon Jay (stress reaction in the left wrist) began an injury-rehab stint for Memphis on Friday night. ... The Giants placed RHP Matt Cain (right elbow nerve irritation) on the 15-day disabled list and recalled RHP Chris Heston from Triple-A Sacramento. Heston immediately went into Cain's spot in the rotation. He'll start Sunday's series finale.