Washington's Ryan Zimmerman crossed home plate with the winning run in the bottom of the 11th inning after a throwing error by Astros third baseman Jimmy Paredes to give the Nationals a 4-3 win in Friday's opener.
Zimmerman worked a one-out walk against Lucas Harrell (0-1) and Michael Morse followed with a walk to put the winning run at second. Wilton Lopez entered the game to face Jayson Werth, who slapped a grounder to Paredes. Paredes rushed his throw to second and the ball rolled into right field, allowing Zimmerman to score.
Zimmerman added a two-run homer and Chris Marrero chipped in an RBI double for the Nationals, who snapped a two-game skid and won in walkoff fashion for the 12th time this season.
Tyler Clippard (3-0) tossed two innings in relief to earn the win.
Lannan, a 6-foot-4, 215-pound native of Long Beach, N.Y., was a nine-game winner in each of his first two major-league seasons in 2008 and 2009, then won eight times in 25 starts with the Nationals last season.
He got to win No. 9 this year in his most recent start on Sept. 5, when he pitched 5 1/3 innings of five-hit, one-run ball on the way to a 7-2 defeat of the Los Angeles Dodgers.
The win snapped a four-start skid for Lannan, who'd been beaten by Philadelphia twice, Arizona and Atlanta since last winning on Aug. 2.
He's 5-6 in 15 home starts overall in 2011 and 3-2 in six career matchups with the Astros to go along with a 4.33 earned run average in 35 1/3 innings.
Houston counters with 32-year-old lefty Wandy Rodriguez, who's won double- digit games in four of seven seasons, including his last three in a row.
The Dominican got to No. 10 this year with a 7-4 defeat of the Pittsburgh Pirates on Aug. 29, when he went seven innings and allowed three runs on four hits with a season-high 13 strikeouts.
He's alternated wins and losses in his last eight decisions, following up the defeat of the Pirates with a 4-0 loss to Milwaukee after allowing six hits and three runs in six innings on Sept. 4.
Rodriguez is 4-1 in his career against the Nationals in eight starts, walking 18 batters and striking out 37 in 47 innings.
The Astros took two of three over the Nationals on July 18-20, but lost three of four in Washington a season ago.