Webb will look to become baseball's first five-game winner on Tuesday when his Arizona Diamondbacks try to improve upon the major leagues' best record when they close a two-game series with the visiting San Francisco Giants.

Webb set career-highs in wins (18) and strikeouts (194) and had the second-best ERA (3.01) in baseball last season, but the brilliant season of San Diego's Jake Peavy kept the Diamondbacks' right-hander from winning back-to-back Cy Youngs.

Four starts into 2008, Webb (4-0, 1.86 ERA) looks like he has something to prove, and Arizona (14-5) is once again reaping the benefits. Webb has gotten better each time out, and he allowed one run on three hits in eight innings in his most recent outing, a 4-1 Diamondbacks win over San Francisco (8-12) on Wednesday.

"Everything has been clicking right now," Webb told the team's official Web site. "I know it isn't going to go like this all year."

Webb is 3-0 with a 2.25 ERA in his last five starts against San Francisco. Right fielder Randy Winn has had some success against Webb in his career, going 12-for-30 (.400).

But Webb isn't the only Arizona starter who is off to a strong start. The Diamondbacks' rotation has a 12-3 record and a major-league best 2.60 ERA through 19 games.

Micah Owings joined Webb as one of three pitchers to start 4-0 on Monday, when he went 6 1-3 innings, allowing two runs in Arizona's 4-2 win in the series opener.

The Diamondbacks improved to 13-3 against NL West opponents.

"A lot of good baseball players in here," said left fielder Eric Byrnes, who hit a two-run homer. "I don't think this is an aberration the way we're playing. I think this is the type of team that this is."

In addition to having the best starting pitching in baseball, Arizona also has scored the most runs (120) in baseball. The Diamondbacks haven't scored fewer than four runs since the third game of the season.

Six different players have at least three home runs, and those six all have at least 10 RBIs, led by third baseman Mark Reynolds' six homers and 19 RBIs.

Scoring runs has been the big problem for the young Giants' offense - they only have 63 through 20 games, tied for the fewest in baseball.

"It's just growing pains you have with young players," manager Bruce Bochy said.

What San Francisco has been getting recently, however, is quality starting pitching. The Giants' last three starters have a combined 1.50 ERA.

Barry Zito (0-4, 4.50) certainly hasn't lived up to the seven-year, $126 million contract he signed with San Francisco in January 2007, but he hasn't pitched as poorly as his record would indicate in 2008.

No pitcher in baseball has received worse run support, as the Giants have scored just only three runs in the four starts Zito has made.

"Sometimes, when you haven't had a win, you have a tendency to start pressing a little bit. Going against Webb is not an easy task," Bochy said. "We know it would be big if we could get Barry a win and let him relax a little bit."

Zito was on the mound Wednesday when Webb improved to 4-0. The Giants' left-hander went six innings, allowing four runs - three earned - but he walked five.

Zito is 2-4 with a 3.00 ERA in six career starts against Arizona.