ST. LOUIS -- Perhaps it could have been predicted that this game was likely to hinge on what happened early. A pair of pitchers, both of whom have endured regular first-inning frustrations, each found themselves in first-inning trouble again.
Barry Zito emerged unscathed. Lance Lynn did not.
And indeed, that would be the difference in the Cardinals' 4-2 loss to San Francisco at Busch Stadium. Gone was St. Louis' four-game winning streak, as well as an opportunity to gain National League Central ground on the Reds and Pirates, both of whom also lost Tuesday.
"One swing, one pitch," Lynn said, "cost me the game."
The outcome supported such an assessment. That pitch -- a 2-1 cutter to All-Star catcher Buster Posey with two on base -- hushed a sellout crowd as the ball landed over the wall in center, giving the Giants a first-inning cushion.
It was Posey's 18th home run this year and his fifth in his last six games.
"I wasn't trying to give him anything to hit there," Lynn said. "It was just a bad pitch."
Preceding the homer were back-to-back leadoff singles, one of which second baseman Skip Schumaker nearly stopped.
"I probably should have made that play," Schumaker said. "[Things] might have been different."
Instead, the Giants exploited Lynn's tendency to struggle at the get-go. Thirty-four percent of the runs Lynn has allowed this year have come in the opening frame. Opponents are batting .322 against him in it.
Well aware of the trend, Lynn chose to alter his pregame routine Tuesday. He went to the bullpen earlier than usual to throw his warmup pitches, a change that, based on results, only left Lynn wondering afterward what he needs to attempt next.
"It's been horrible," Lynn said of the first inning. "It's just one of those things where it seems like all my bad luck happens in the first. It's always my high-pitch inning. It's something that I'm aware of and trying to fix. I'm also a little amped up and balls are sometimes up."
Added manager Mike Matheny: "I think some of that is just trying to get his rhythm, which is typical of a young pitcher. He has pitched out of it most of the time. Today, you make a mistake to Posey and he's going to make you pay."
For as many first-inning problems as Lynn has endured, Zito has actually been worse. Zito, who owns a 3.55 ERA from the second inning on, entered Tuesday having given up 20 runs in the first inning of his 21 starts.
Zito retired St. Louis' first two hitters, but consecutive hits put two in scoring position for third baseman David Freese. This time, Zito found a way out.
Freese grounded out to short to end the first.
"I was looking to come out here and be aggressive," said Zito, who earned his first win at Busch Stadium. "That was my game plan. But when we go up three in the first, it's even more important to come out, throw strikes and put the pressure on their offense."
Zito was by no means unhittable for the rest of his 6 2/3-inning start, but he managed to concentrate trouble in his favor.
Allen Craig alone provided the offensive highlights, as he twice took Zito deep. The homers -- one with two out in the third, the other with two out in the fifth -- gave Craig his fourth career multi-homer game.
"You just have to keep things simple and not miss the pitches that he gives you," Craig said. "You can't miss the good ones that he gives you."
The Cardinals stranded two runners in the first, fourth and sixth, though in each of those cases the second hit of the inning came with two outs. Yadier Molina's two-out, eighth-inning double went for naught as well.
"It's tough to swallow when you had multiple opportunities to capitalize," Schumaker said. "But it's not out of the question that he can do that to a team."
In the meantime, Lynn was encountering his own trouble during a laborious, six-inning start. Though he limited the Giants to only one additional run after the first, there were baserunners swarming behind him all night.
It took a double play to get out of a bases-loaded jam in the second. Another two runs were saved in the fourth, when center fielder Jon Jay made a basket catch as he tracked a ball to deep center.
One inning later, Carlos Beltran's catch against the right-field wall ended the frame with one on base.
The only other run Lynn did allow was one he was actually positioned to erase. In the sixth, Lynn caught Brandon Belt breaking toward second too early and threw to first baseman Craig. Craig's throw to second base hit Belt in the back.
Matheny briefly argued that Belt put himself into position to be hit by the throw when he veered onto the edge of the grass while running. His argument was countered by the umpire, who said, "It doesn't matter where he goes, as long as he doesn't turn around."
Joaquin Arias then drove Belt in with a one-out double.
Lynn's 105-pitch outing was the latest in a stretch of see-saw results for the right-hander. In five of his last nine outings, Lynn has given up at least four earned runs. He's allowed at least one first-inning run -- and a total of 13 -- in each of those games.
He's allowed two or fewer runs in the other four starts during that span. Altogether, Lynn's ERA has climbed from 2.42 to 3.52.
"I think you're seeing a young pitcher that is getting more looks at," Matheny said. "We still had a chance to win today. He's been doing that pretty consistently for us."
Jenifer Langosch is a reporter for MLB.com. Read her blog, By Gosh, It's Langosch, and follow her on Twitter @LangoschMLB. This story was not subject to the approval of Major League Baseball or its clubs.