Ian Kennedy allowed one hit over eight innings to earn his 20th victory and Justin Upton led off the sixth inning with his 31st home run as the Arizona Diamondbacks posted a 1-0 victory over the visiting Pittsburgh Pirates on Monday.

Upton finished with two of Arizona's five hits as the Diamondbacks moved 5 1/2 games ahead of idle San Francisco in the National League West and reduced their magic number for clinching the division to four.

Kennedy (20-4) struck out 12 and walked one en route to joining Detroit's Justin Verlander as the only 20-game winners in the major leagues. The Cy Young Award candidate has won five straight decisions and 12 of his last 13.

Kennedy retired the first eight batters he faced before yielding a single to Jeff Karstens and set down the final 14 before giving way to J.J. Putz, who allowed a one-out triple by Alex Presley before notching his 21st consecutive save and 42nd in 45 chances this season.

Upton broke his bat on a 3-0 offering from Karstens and muscled the ball over the wall in left field. Pittsburgh argued a fan interfered on the play, but the homer was upheld after video review.

Karstens (9-9) surrendered one run and five hits over six innings but fell to 1-5 in his last 10 starts. The righthander has allowed one run in 22 career innings against Arizona.

Arizona moved within one game of Milwaukee for the No. 2 seed in the NL playoffs.