ST. LOUIS -- A day after teammate Chris Carpenter helped his cause for a second Cy Young Award by clinching the NL ERA title, Cardinals starter Adam Wainwright took to the Busch Stadium mound hoping to make one final impression on any undecided voters.

After leaving with a comfortable five-run lead in the seventh inning, the hardware hopeful appeared to do just that.

But just nine outs away from becoming the Major League's only 20-game winner and the first St. Louis pitcher to achieve the feat since Carpenter in 2005, Wainwright watched as history -- and possibly his first Cy Young Award -- slipped right out of his hands.

Reliever Kyle McClellan gave up four runs in one-third of an inning and allowed two more of Wainwright's runs to score as the Brewers scored six runs in the seventh and five more in the eighth to come from behind and deal the Cardinals a stunning and potentially hurtful 12-6 loss on Friday night.

St. Louis could have tied Philadelphia for the National League's second-best record after the Phillies lost, 7-2, to Florida earlier Friday but instead now appears likely to open the playoffs next Wednesday on the road.

"This is right there with the most disappointing, toughest regular-season losses that I can remember," said manager Tony La Russa. "It was an important win for us if we can get it with what was at stake. Adam has been so great all year long. I'm really, really disappointed."

After needing 130 pitches to get through eight grueling innings in his last start in Colorado on Saturday, La Russa decided to lift Wainwright after just 90 pitches against Milwaukee on Friday night.

With a 6-1 lead and Wainwright on the brink of becoming the 48th 20-game winner in Cardinals history, La Russa pulled his starter despite the eighth- and ninth-place hitters coming up.

"I could taste it," Wainwright said of the 20th victory. "And that may be why the first two guys got on, I got a little ahead of myself there. I started thinking a little too far forward, I think. We win that game 99 times out of 100. Our bullpen has been great this year and I pride myself on not giving away leads. I'll take my chances with a six-run lead all day."

McClellan, who entered as Wainwright left to a standing ovation, picked a bad night to have one of his worst outings of the season. The righty walked Jason Kendall -- who entered the at-bat hitting .242 -- to load the bases with no outs before striking out pinch-hitter Mat Gamel. But three consecutive RBI hits, capped by a two-run double by Ryan Braun, tied the game and took away the potential historic victory for Wainwright.

A crowd that gave Wainwright a standing ovation and curtain call just minutes earlier booed heavily as La Russa went to the mound and took the ball from McClellan.

"He was the first one there to say 'I could care less about that, get your head ready and get ready for the playoffs,'" McClellan said of Wainwright. "That shows the kind of person and leader that he is. But it's just disappointing to come in and see him there. It's hard, because as hard as he's worked and fought and everything he has done, it's a shame it came down to my lack of execution.

"It's going to be a tough night."

The Brewers took the lead on a wild pitch by Jason Motte before turning the game into a laugher with five more runs in the eighth. The Cardinals needed seven pitchers to get six outs in the seventh and eighth innings, and St. Louis relievers threw five wild pitches, four of which allowed runs to score.

"It just snowballed," La Russa said. "It was ugly."

Wainwright may have been denied the coveted 20th win but still has a strong case for his first Cy Young Award. The righty finished the season with a league-high 19 wins, had an ERA of 2.63 and his 233 innings pitched also lead the National League.

But when you factor in that four of Wainwright's eight losses came by scores of 1-0, 2-0, 2-1 and 3-2, the righty -- as he did on Friday night -- deserved even better.

"It's cool to have a notch on the belt, but it's more important that [McClellan] realizes it's just one outing," Wainwright said. "He's going to have to be good for us in the playoffs if we want to do well. It's one of those things where we lost the game and that stinks, but he can really grow from this and get better. And I believe he will.

"There were a lot of things that sent that game the other way. Kyle's been great for us. He just had a bad night."

The Cardinals gave Wainwright some rare offensive support when Matt Holliday and Ryan Ludwick singled in runs in the first to make it 2-0. RBI doubles by Albert Pujols and Ludwick and a run-scoring single by Yadier Molina made it 5-0 in the third before Wainwright tripled and scored in the fourth to make it 6-0.

But instead of celebrating what should have been a memorable win for Wainwright, the Cardinals instead emptied their bench in the ninth inning like a Spring Training game and suffered what might be their worst loss of the season.

"That felt like a playoff game for us," said Braun. "We're obviously not playing for much; [the Cardinals] are on to bigger and better things. But for one night, it felt pretty good."

Not in the Cardinals' clubhouse.