ST. LOUIS -- For a moment, it looked like the Washington Nationals had frittered away a potential run-scoring chance in the fourth inning.

But manager Dusty Baker's successful challenge put Washington back in business, and as things turned out, it led to the big inning they needed to start a 10-game road trip off with a win.

Scoring four runs in the fourth to take the lead for good, Washington won for just the eighth time in 34 games in St. Louis since moving from Montreal before the 2005 season, edging the Cardinals 5-4 Friday night at sold-out Busch Stadium.

"Replay definitely helped us tonight big-time," Baker said.

After singling to lead off the fourth, Anthony Rendon attempted to swipe second with one out as Ryan Zimmerman took a 2-1 pitch outside. Catcher Eric Fryer's throw appeared to be late, but second baseman Jedd Gyorko slapped a quick tag on Rendon and Doug Eddings called him out.

However, Baker asked for a replay and it revealed what Eddings didn't see from his angle -- that Rendon got his hand to the bag just ahead of the tag.

"I was hoping it wouldn't be conclusive," Fryer said of the replay, "but it was. It was pretty easy to tell on the replay that he was safe."

With the call reversed, the Nationals (15-7) pressed their attack. Zimmerman walked, Daniel Murphy singled to right to score Rendon and Jayson Werth's sacrifice fly to right brought Zimmerman home for a 3-2 advantage.

Danny Espinosa then pounced on a 1-2 mistake by Leake, attacking a fastball that swayed toward the plate's middle and driving it into the St. Louis bullpen behind the right-center-field wall for his first homer of the year.

Fryer said Leake tried to repeat a pitch that beat Espinosa for the first two strikes of the at-bat.

"He tried to catch the outside corner and it just caught too much of the plate," Fryer said. "Espinosa had a really good at-bat."

It proved to be the killing blow for Leake (0-3), who was charged with five hits and five runs in seven innings with a walk and two strikeouts. Signed for five years and $80 million as a free agent in December, Leake has permitted at least four runs in all five starts.

Meanwhile, Stephen Strasburg (4-0) shrugged off a shaky first inning in which he was touched for an RBI double by Matt Holliday and a run-scoring single by Matt Adams on consecutive pitches.

Strasburg kicked it into high gear in his third pass through the order, fanning seven of eight hitters. After whiffing Matt Carpenter to strand two men in the fourth, Strasburg struck out the side in the fifth with a Holliday single mixed in, then whiffed the side in the sixth.

"He's no longer a kid," Baker said of Strasburg. "Now he's a pitcher. He knows what to do, he doesn't need anyone to guide him through the game. He kept his composure and we got him a lead, and he got locked in."

Strasburg scattered eight hits in his seven innings, walking one and fanning nine for his first win in five career starts against the Cardinals (12-11).

Reliever Blake Treinen coughed up a two-run homer to Matt Adams in the eighth to draw St. Louis within a run. Closer Jonathan Papelbon hinted at a tightrope ninth when he walked pinch-hitter Kolten Wong to start it, but erased it with a 4-6-3 double play off pinch-hitter Brandon Moss' bat and retired Carpenter on a grounder for his eighth save in nine chances.

Michael Taylor ended Washington's 22-inning scoreless streak with a leadoff homer on the game's third pitch. It was an omen of things to come for the Nationals, who used the replay review to craft the big inning that gave Strasburg the cushion he needed.

"I was kind of searching for things a bit early, but the guys gave me a chance and scored some runs," he said.

NOTES: St. Louis C Yadier Molina got the night off after starting in 21 of the team's first 22 games, with Eric Fryer picking up his second start. ... One big reason for Washington's fast start? Its defense has committed an MLB-low seven errors and ranks third in defensive efficiency, converting 74 percent of batted balls into outs. They were 21st last year at 70 percent. … Cardinals SS Aledmys Diaz needs one more hit to join Albert Pujols as the franchise's only rookies to bang out 30 hits in April.