ESPN MLB

PHOENIX -- Just below a sign that said "We Believe," a speaker blared Journey's "Don't Stop Believin" across Arizona's clubhouse.

A setup? Possibly.

Either way, there's not much doubt in these Diamondbacks.

Ian Kennedy pitched eight dominant innings to become the NL's first 20-game winner, Justin Upton hit a broken-bat homer and Arizona moved a step closer to its first NL West title in four years with a 1-0 victory over the Pittsburgh Pirates on Monday night.

"This is the most important part, which is winning games, taking away that magic number and clinching here pretty soon," said Kennedy, who joined Detroit's Justin Verlander as the only 20-game winners in the majors.

Their lead quickly dwindling over surging San Francisco, the Diamondbacks needed a boost to start a nine-game, season-ending homestand.

They certainly got it from Kennedy.

The built-like-a-boulder right-hander had the Pirates guessing all night, giving up one hit -- to the opposing pitcher, no less -- while matching his career high with 12 strikeouts to become the fourth Diamondbacks pitcher to reach 20 wins.

Kennedy gave the Diamondbacks what has become expected of him.

Upton provided an improbable shot, leading off the sixth inning with his 31st homer off Jeff Karstens (9-9), a drive to left that snapped off his bat and was later upheld by a video review.

J.J. Putz closed out the two-hitter with his 42nd save -- and 21st straight -- by stranding Alex Presley at third to drop Arizona's magic number to four over the idle Giants.

"We can't relax now," Upton said. "There's no point in relaxing. We have to finish it off."

The Diamondbacks were firmly in control of the NL West just a couple of weeks ago, leading by 9½ games with 16 left. They entered the three-game series against Pittsburgh a little more white knuckled thanks to San Francisco's eight-game winning streak and a three-game skid of their own that ended with a win over San Diego on Sunday.

Arizona hoped to climb closer to its first division crown since 2007 against the fast-fading Pirates, who led the NL Central in late July but fell 22 games out of first after a 15-1 loss to the Dodgers on Sunday.

The Diamondbacks figured to have a good shot with Kennedy on the mound.

Arizona's ace has become a legitimate Cy Young Award contender by allowing one run or less 16 times on his way to a 2.88 ERA. The right-hander had gone 11-1 with a 2.42 ERA in his previous 13 starts and had a 1.69 ERA while winning his previous six outings at Chase Field.

Kennedy overmatched the Pirates from the start, retiring the first eight hitters before Karstens, an .087 hitter, lined a single through the left side. Kennedy seemed to get stronger after that, striking out the final six batters he faced to join Randy Johnson and Curt Schilling (2001-02) and Brandon Webb (2008) as the only 20-game winners in Diamondbacks history.

"Our guys were trying to stay after it all night against him," Pirates manager Clint Hurdle said. "He was very, very sharp."

Karstens wasn't bad, either. He had missed two starts in September due to shoulder fatigue, thanks in part to a career-high 156 1/3 innings, and was just OK in his return last Tuesday against St. Louis.

The right-hander looked good against the Diamondbacks, scattering four hits over the first five innings before Upton hit his bizarre homer.

Despite hitting the ball squarely, Upton was left with just the handle of his bat in his hands as the rest of it flew to the dirt at third base. The ball sailed just out of the reach of Presley in left after a fan touched it on the way down, but the homer was upheld after a quick video review by the umpires.

Karstens allowed five hits before being lifted for a pinch-hitter after six innings.

"I wanted to go back out," Karstens said. "It was a 1-0 ballgame. I felt like I still had good stuff."

Just not as good as Kennedy's.

Game notes

Arizona 3B Ryan Roberts turned 31 on Monday. ... Karstens has allowed 22 homers this season, accounting for 26 of the 69 runs he's allowed. ... Arizona RHP Daniel Hudson will pitch the second game of the series after having his four-game winning streak snapped by the Dodgers in his last outing. He's tied for fifth in the NL with 16 wins. ... RHP Charlie Morton, who will start for Pittsburgh on Tuesday, has lost four straight decisions since his last win on Aug. 8 at San Francisco. It will be his sixth attempt at winning his career-best 10th game.