WASHINGTON (Reuters) - Roy Halladay, considered one of the game's top pitchers, struck out nine batters for a win in his debut on Monday with a Philadelphia Phillies team seeking a second World Series championship in three seasons.

After a shaky first inning where he allowed a single, a double, a stolen base and a run, Halladay settled down and never gave the Washington Nationals a chance to build on an early one-run lead as Philadelphia went on to win 11-1.

In a game where the ceremonial first pitch was thrown out by U.S. President Barack Obama in front of a near-capacity crowd at Nationals Park in Washington D.C., the home team were not able to build on their early lead.

The 32-year-old Halladay, a six-times all star and the 2003 American League Cy Young Award winner as top pitcher, allowed six hits and two walks through seven innings.

After giving up a double to Ivan Rodriguez to start the second inning, Halladay, with the help of some double plays, faced the minimum number of batters until the seventh inning.

The Phillies, who acquired Halladay during the off-season from the Toronto Blue Jays for three prospects, blew the game open with a five-run fourth inning.

Ryan Howard hit a two-run homer while Shane Victorino and Halladay each knocked in a run. Victorino then scored on a Placido Polanco sacrifice fly.

The Phillies followed their two-run sixth inning with four more in the seventh as Polanco belted a two-out grand slam in a game he finished with a career-high six RBI.

The Nationals, who led the majors last year with 103 losses, fell to 2-4 in home openers since baseball returned to Washington in 2005.

Philadelphia are coming of two straight World Series appearances. The Phillies won it all against Tampa Bay in 2008 but lost to the New York Yankees last year.

(Writing by Frank Pingue in Toronto; Editing by John Mehaffey)