PHOENIX -- It had not been a good month of August for Sean Marshall.

Although he had picked up a win in his previous start, Marshall's ERA in four outings this month was 8.50 and he had yet to throw a quality start entering Friday night's game.

The Cubs southpaw got back to his form from earlier in the season at Chase Field, as the Cubs beat the D-backs, 6-2, in front of 36,700 in the opener of this three-game series.

"They came out aggressive, and it was to my advantage because they popped some balls up and got me some quick innings," said Marshall. "I just kind of used that momentum to carry me deeper into the game."

"I felt like I had command of all my pitches today and made some pitches when I needed to. I can take a lot of positives from it and keep working."

In a game that was tight through eight innings, Marshall (7-6) looked more like the pitcher that had posted a 3.10 ERA in 12 starts entering this month.

The southpaw was never in that much trouble in this one, as he allowed just three baserunners in six innings of work. He allowed two hits, walked one and struck out five.

"He did a real nice job," said manager Lou Piniella. "He pitched six really good innings of baseball."

At times the crowd seemed as if it were dominated by Cubs fans, especially as the Chicago bats jumped on D-backs starter Micah Owings early. Derrek Lee and Mark DeRosa both had first-inning RBI singles to give the Cubs an early 2-0 lead.

The only blemish on Marshall's linescore occurred in the third. D-backs starter Micah Owings (6-7) was coming off of a game where he had homered twice and driven in six in his last start, and he took Marshall deep for a solo shot that cut the lead in half.

Pineilla even joked before the game that they "might have to walk [Owings] intentionally" with a laugh, but he wasn't laughing anymore after Owings' blast.

"We knew this guy could hit," said Piniella. "Marshall was trying to sink the ball away, and it stayed in the middle. That's impressive. He can swing it."

In 46 at-bats this season, Owings is hitting .283 with four homers and seven extra-base hits.

"That guy just seems to be on fire right now," said Marshall.

Still clinging to that one-run lead, a key sequence took place in the bottom of the fifth. D-backs catcher Chris Snyder led off with a double and advanced to third on a wild pitch. With one out and Snyder still on third, Justin Upton came to the plate.

Piniella brought the infield in halfway because "Snyder doesn't run all that well."

Upton hit a ball in the hole at short, but Ryan Theriot made a diving grab and got up and threw to the plate, where Jason Kendall tagged Snyder out and kept the Cubs' lead.

"That was a great dive and accurate throw, and Kendall did a great job of blocking the plate," said Piniella.

"Upton flies, so I may not have had him at first," said Theriot. "On my dive, I kind of peeked to see where Snyder was so I knew I had a shot."

From there the Cubs added a run in the sixth and then broke the game open with three in the ninth -- including a hit for Craig Monroe in his first at-bat as a Cub -- as the bullpen held the D-backs bats in check.

There were some anxious moments in the bottom of the ninth, as the D-backs plated a run and had enough momentum to have the tying run in the on-deck circle with two out. That brought on Ryan Dempster, who got Upton to fly to right for his 21st save.

Though the Cubs wouldn't really admit to it, they were certainly looking at the scoreboard to check the score of the Brewers' game, one Milwaukee lost to fall 1 1/2 games behind the Cubs in the National League Central.

"It's easy to watch the scoreboard at this time of year, but you better concentrate on what you're doing or you could lose [the lead] real quick," said Piniella.