BALTIMORE (AP) -Desperate for a victory against their neighbors to the north, the Washington Nationals got just what they needed from their most reliable pitcher and most productive slugger.

John Lannan took a four-hitter into the eighth inning, Adam Dunn hit a colossal home run, and the Nationals defeated the Baltimore Orioles 5-3 on Sunday to avoid a three-game sweep.

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Willie Harris also homered for Washington, and Josh Willingham had three hits and scored twice. By winning their interleague finale, the Nationals generated a bit of optimism for their upcoming NL East series against the Marlins.

``It was good to get that W and get on the plane and go down to Florida happy,' Harris said. ``Hopefully get some of that momentum for (Monday).'

Lannan (5-5) gave up three runs and six hits in 7 1-3 innings. The left-hander went 3-0 with a 2.19 ERA in June; he had never before won three games in one month.

In the process, Lannan helped establish himself as the leader of a young starting rotation that owns a collective 15-27 record.

``I think he is, without a lot of hoopla,' manager Manny Acta said. ``I think the other kids realize that. He leads by example. He works very hard.'

Lannan has allowed three earned runs or fewer in 12 of his last 13 starts.

``Last year I would have a quality start, make it two in a row, then I would kind of fall off,' he said. ``I'm finally finding some consistency, which is great.'

Mike MacDougal worked a perfect ninth for his third save.

Washington trailed 1-0 in the fourth inning before Dunn connected on an 0-2 pitch from rookie David Hernandez (1-2) following a double by Ryan Zimmerman. The ball soared over the right-field scoreboard, landed on Eutaw Street - an estimated 442 feet from the plate - before hitting the B&O Warehouse on one bounce.

It was the 50th home run to hit Eutaw Street in the 18-year history of Camden Yards, the first by a Nationals player. Dunn, who came to the plate locked in an 0-for-10 skid, has 19 homers this season and 297 for his career.

For Dunn's teammates, the shot was impressive as it was important.

``It was unbelievable. I didn't even tell him good job or way to swing. You don't have to say anything to that,' Harris said. ``It was just good for us to get that lead right there and for Lannan to go out and pitch with the lead.'

Washington added a run in the sixth when Elijah Dukes singled in Willingham, who scored after rookie catcher Matt Wieters tried to apply a quick tag and dropped the throw from Adam Jones in center.

``It was one of those things were I knew the guy was dead and Jones made a great throw,' Wieters said. ``It hit the glove and popped out.'

Harris homered off Chris Ray in the seventh, and Willingham made it 5-1 in the eighth when he doubled, stole third and continued home on a throwing error by Wieters.

In 21 games as Baltimore's catcher, Wieters has caught only two of 15 would-be stealers and has made three errors.

Recalled from Triple-A Norfolk before the game, Hernandez allowed three runs and eight hits in 5 2-3 innings. The homer to Dunn was the first against the right-hander in four big-league appearances.

``I was trying to elevate a fastball and I just put it right down the middle,' Hernandez said. ``I put it on the tee for him. That's what he does with mistakes - he hits them out.'

The Orioles went up 1-0 in the second inning when Nolan Reimold drew a leadoff walk, took third on a double by Luke Scott and scored on a single by Oscar Salazar.

Baltimore didn't get another runner into scoring position until Robert Andino led off the eighth with a double. Ty Wigginton followed with a single to chase Lannan, and Nick Markakis hit a sacrifice fly off Joe Beimel before Aubrey Huff hit an RBI single. Huff, however, was thrown out trying to stretch the hit into a double.

``We made too many mistakes in a lot of different areas,' Orioles manager Dave Trembley said. ``That was the game.'

NOTES: Washington finished 2-4 against Baltimore and 7-11 against the AL. The Orioles were 11-7 in interleague play. ... Orioles 2B Brian Roberts received a day off after going 0 for 10 in his previous three games. ... The 10-game hitting streak of Washington's Cristian Guzman ended with an 0-for-5 performance.