SAN FRANCISCO (AP) -- Ian Kennedy watched Cody Ross ' leadoff home run sail into the seats in left field and quickly told himself to regroup.
After that, Arizona's ace dominated the defending champs.
Kennedy struck out seven in eight innings to win his fifth straight start and the Diamondbacks beat the San Francisco Giants 5-2 on Monday night in a matchup of the NL West's top two teams.
Kennedy (13-3) moved into a share of the NL lead for wins with Phillies ace Roy Halladay and Dodgers All-Star Clayton Kershaw . Kennedy's big night also helped Arizona pull within one game of the World Series champion Giants in the division race.
"I think this series is very important," Kennedy said. "You want to stay close. You don't want to let them creep away."
The Giants, who dropped their fourth straight after being swept in Cincinnati over the weekend, have been alone in first place in the West since June 25. They couldn't do enough on an off night by All-Star Matt Cain (9-7).
Ross and Aubrey Huff each hit solo home runs for San Francisco, Ross leading off the first and Huff on an 11-pitch at-bat in the seventh, but Kennedy still has allowed three or fewer runs in seven of his eight career starts against San Francisco.
Kennedy allowed six hits, two runs and walked one. After Ross' homer, he retired 10 straight and 17 of 19 before Carlos Beltran 's sixth-inning triple. J.J. Putz pitched the ninth for his 25th save in 29 chances, getting pinch-hitter Aaron Rowand on a game-ending liner to right with two runners on.
"It was nice to come in and get Game 1," D-backs leadoff man Willie Bloomquist said. "Hopefully, it kind of sets the tone for the rest of the series that we beat one of their horses today and our guy pitched pretty dang good, too."
Beltran, San Francisco's newly acquired slugger, went 2 for 4 in his home debut and received a warm standing ovation from the sellout crowd at AT&T Park when he batted in the first inning. Beltran grounded into an inning-ending double play in the eighth with two runners aboard and his team trailing by three runs. Beltran had gone 2 for 17 in his first four games since the New York Mets traded him last Thursday. The Giants are 1-4 since Beltran arrived.
Shortstop Orlando Cabrera , another Giants newcomer, went 1 for 4.
The Diamondbacks have high expectations for a new guy of their own - and first baseman Paul Goldschmidt quickly showed why.
Hours after Arizona announced it had purchased the contract of the promising infielder from Double-A Mobile, he delivered a sharp single to right field in the second inning against Cain in his first major league at-bat.
Goldschmidt, batting seventh in the lineup, will get the ball as a souvenir, too, after Beltran retrieved it and threw to the infield.
The 23-year-old Goldschmidt batted .306 with 30 home runs and 94 RBIs in 103 games for Mobile. At the time of his promotion, he led all minor leaguers in home runs and RBIs and ranked third with 82 walks.
Cain was tagged for five runs in the sixth inning, the first time he has given up five in an inning since doing so in the bottom of the first on Sept. 11, 2008, at San Diego. The right-hander has only allowed more than five runs once this year.
"We made some good pitches. We just didn't get those couple of groundballs to go at guys," Cain said.
The Giants kicked off an important 10-game homestand with another tough loss for their second four-game losing streak of the year. Five games is the team's longest. San Francisco, outscored 20-5 while being swept by Cincinnati, has had three games trimmed off its lead in as many days. The Giants managed just a .184 batting average during their 2-4 trek through Philadelphia and Cincinnati.
Kennedy certainly kept them in check.
"He's been outstanding," manager Kirk Gibson said. "He did exactly what this club needed tonight and pretty much commanded the game."
Arizona won for only the third time in the last 13 meetings between the rivals and is 3-7 in this year's series. The D-backs won at AT&T Park for the first time in four tries in 2011.
"They've owned us," Kennedy said. "We've got to keep this momentum."
San Francisco manager Bruce Bochy is looking for someone to take hold of the leadoff spot, and Ross might be making his case after his first audition. He had two hits with the homer and scored a run. Ross started in center field in place of the struggling Andres Torres .
NOTES: It was Ross' second career leadoff homer and first since 2008 with Florida. ... The Giants placed struggling LHP Barry Zito on the 15-day DL with a recurrence of his earlier right foot sprain that landed him on the DL previously. Zito lost his third straight start Sunday at Cincinnati. San Francisco needed to make room for the return of LHP Jonathan Sanchez , who will start Friday night against the Phillies. ... Giants LHP starter Madison Bumgarner celebrated his 22nd birthday.