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The surging Washington Nationals pay a visit to Citizens Bank Park Monday to begin a key four-game series with the Philadelphia Phillies.

Washington enters the series opener in search of a fifth straight win after sweeping the San Diego Padres over the weekend. The Nationals trail Atlanta by four games in the National League East standings, while Philadelphia sits 7 1/2 games back.

Ryan Zimmerman belted a grand slam and Anthony Rendon tacked on a two-run home run in Washington's 11-7 win over the Padres on Sunday. Stephen Strasburg fanned nine batters in six innings to pick up the victory.

"I mean, we've had some bumps in the road with injuries and stuff, but the character and makeup of this club is great," Nats manager Davey Johnson said. "Even through the rough times, I haven't seen anybody getting overly concerned. Everybody's starting to swing the bats and that's been a big concern, [and] the bullpen's straightened out."

Philadelphia has also started to pick things up of late. The Phillies won two out of three at NL-leading Pittsburgh last week, and then followed that up with a series victory over the Braves this weekend.

Domonic Brown tripled and homered in Sunday's 7-3, series-clinching win over the Braves. Brown, who was voted to his first career All-Star game, has 23 home runs and 62 RBI on the season. He has gone 8-for-21 with two home runs and four RBI over his last five games.

"We have to go right now," Brown said of his team's urgency level. "We're definitely fighting and battling. Hopefully we can win the series against the Nats."

The Phillies are without slugger Ryan Howard and he is out with a knee issue. Howard is expected to undergo an MRI on his knee to determine what's wrong.

Taking the hill for the Phillies Monday is John Lannan, who has gone 1-2 with a 4.50 ERA in four starts since returning from a two-month layoff with a quad injury.

The left-hander gave up four runs on a season-high 11 hits over five innings against Pittsburgh on Wednesday as the Phillies lost, 6-5. Lannan also walked three batters.

Dan Haren will come off the disabled list (shoulder) to make the start for Washington. The veteran right-hander is 0-6 with a 7.01 ERA over his last eight starts and has been hit hard by the Phillies twice this season. Philadelphia is the only team in the majors Haren has yet to beat in his career.

"With the rest, I was going to feel fresh. Hopefully that's what I needed to get back doing some good things in helping the ballclub," Haren said after pitching a simulated game last week. "I'm getting the ball to where I usually do. Before, the last couple of games, I always long tossed between starts and before the game, and it was a struggle to get the ball there. My arm is moving faster."

The teams have split the six meetings this season. Most recently, the Phillies took two of three from the Nats at home last month.