SAN FRANCISCO — Just about the only certainty in the 2010 Phillies' starting rotation is that Roy Halladay will take the ball every fifth day and deliver a performance likely to keep his team in the game.

Now even that has failed the Phillies.

Halladay allowed five runs to the Giants — one more than his season total entering the game — and the Phillies lost for the third time in four games, 5-1, Monday at AT&T Park.

While his team scuffled in Arizona over the weekend, Phillies manager Charlie Manuel observed that his offense was having difficulty scoring after it dominated opposing pitching at will in the first weeks of the season.

The offense was shuttered by San Francisco pitching on Monday, but with Halladay on the mound, the Phillies usually don't need very many runs.

Halladay was victorious in his first four starts as a Phillie and had allowed four runs in 33 innings. He hadn't trailed in a start since the very first innings of the season against Washington.

On Monday night, the Giants scored twice in the first inning and added another run in the second. Mark DeRosa singled home Edgar Renteria and Pablo Sandoval with two outs in the first inning. In the second, leftfielder John Bowker (a .175 hitter coming into the game) led off with a double. Backup catcher Eli Whiteside followed with an RBI double on a 92-mph fastball from Halladay.

In the seventh, Whiteside hit an 0-1 pitch from Halladay off the left-field foul pole. That prompted scattered "overrated!" chants from the Giants fans and action in the Phillies' bullpen.

The summary: Halladay is human.

Until Monday, the stoic Halladay resembled a fearless, strike-throwing machine. Phillies starters had a 3.69 ERA, sixth best in the National League. But take away Halladay's sterling numbers, and the ERA climbed to 4.93 for everyone not named Roy Halladay.

He has carried the staff.

Halladay had no problems throwing strikes Monday — 72 of his 104 pitches were for strikes. But instead of being on the corners like he normally is, some of Halladay's pitches caught too much of the plate, and San Francisco took advantage.

He allowed 10 hits in seven innings. Last season with Toronto, Halladay allowed 10 or more hits in six games.

His offense did nothing to help. Even as Jonathan Sanchez walked five batters in five innings, the Phillies could not capitalize. With the bases loaded in the third, Jayson Werth struck out and Ben Francisco flied out to right.

In the seventh, long after the Phillies had forced Sanchez from the game with a high pitch count, Francisco came up with the bases loaded and two outs again. This time, he whiffed on a 1-2 breaking ball from lefthanded reliever Jeremy Affeldt.

With the Phillies squandering another opportunity, Whiteside delivered the final blow in the bottom half of the inning against Halladay. He left the mound, soon to suffer defeat for the first time in his Phillies career.