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NEW YORK -- John Lannan knows he can't pitch like Stephen Strasburg. What he can do is compete like the Washington Nationals' ace.

Lannan pitched five-hit ball over 5 2/3 innings in his first start since replacing Strasburg in Washington's starting rotation, and the Nationals lowered their magic number for clinching a playoff berth to six with a 2-0 win over the punchless New York Mets on Wednesday night.

"It's definitely strange. There's a lot of strange things going on," Lannan said. "What's going on with Stras, it just hasn't happened before."

Ryan Zimmerman homered in the fourth off rookie Matt Harvey (3-5) and Ian Desmond connected off Jon Rauch in the eighth, helping the NL East leaders complete a three-game sweep and open an 8 1/2-game lead over the second-place Braves heading into a three-game series at Atlanta starting Friday. The Nationals are closing in on the capital's first postseason baseball berth since the original Washington Senators in 1933.

"This was a big series," manager Davey Johnson said. "We need to kind of drive the nail in their coffin over here and come into Atlanta in a good frame of mind."

A native of nearby Long Beach, Lannan (3-0) struck out two and walked one in an 81-pitch outing. Lannan, Washington's opening-day starter in 2009 and 2010, was 9-11 with a 4.30 ERA at Triple-A Syracuse and asked the Nationals to trade him when he was demoted at the end of spring training.

"It all happened for a reason, I guess," he said. "I just don't look back on it with anger or anything. It just is what it is. It happened, and we've just got to move on."

Pitching before about 20 family members and friends, he made his first appearance since a minor league outing on Aug. 30 and improved to 4-1 with a 1.69 ERA against the Mets in his last five starts. Without Strasburg's dominant fastball, the left-handed Lannan relies on changing speeds and control.

"He earned a lot of respect in this clubhouse," said reliever Sean Burnett, who gave up two hits and got two outs in the ninth -- his first appearance since Sept. 2 following a layoff caused by an irritated nerve in his pitching elbow.

Drew Storen came in and retired pinch-hitter Daniel Murphy on a game-ending flyout with runners at the corners for his second save. The surging Nationals have won nine of 11 and 12 of 15.

Harvey struck out 10 in five-plus innings, his latest impressive performance. New York, however, finished an 0-6 homestand and dropped to 65-78, falling 13 games under .500 for the first time since finishing 2009 at 70-92. The Mets have lost eight of nine overall.

They also extended their club-record streak of scoring three runs or fewer to 13 straight home games. They have lost 22 of their last 26 in Queens and have just four wins at Citi Field since the All-Star break -- two fewer than the Nationals.

"It's frustrating for all 35 guys in that clubhouse, besides the players, and the coaches and the front office," Mets manager Terry Collins said. "We set the bar pretty high in April and May. Our club wasn't picked to be very good and we set the bar up there.

"Right now, we just cannot seem to recreate what happened in April."

Washington cut short Strasburg's season last weekend, wanting to protect the 24-year-old right-hander's arm following ligament-replacement surgery on Sept. 3, 2010.

While the Nationals initially intended to make Wednesday's game his final start of the year, they moved up their timetable because they said the limit was weighing on his mind. Strasburg finished 15-6 with a 3.16 ERA in 28 starts with 197 strikeouts in 159 1/3 innings.

Johnson said he might use Strasburg as a pinch-hitter down the stretch. Strasburg is hitting .277 with one homer, four doubles and seven RBIs.

"If I can't use your arm, I'll use your bat," Johnson said. "He's pretty bad now but he's going to get worse because he's going to be chomping."

Johnson said Strasburg keeps asking pitching coach Steve McCatty about what his routine should be.

"Of course, he's been pestering Cat about what he can do," Johnson added. "Can I keep throwing off the mound? No. Can I play catch in the outfield? Yes."

Game notes

Washington has won eight in a row in Queens and 12 of 13. ... Nationals LF Michael Morse missed the game after he was diagnosed with a torn cuff and a bone bruise in his left wrist. ... Players' association head Michael Weiner, undergoing treatment for a brain tumor, was in the Mets' clubhouse speaking with players before the game. ... Zimmerman extended his hitting streak to 16 games and set a franchise record by driving in a run for the ninth straight game, one more than Larry Walker (1993), Moises Alou ('93 and `94) and Vladimir Guerrero ('98). Zimmerman also made several sparkling plays with his glove at third.