PITTSBURGH -- Chicago Cubs manager Joe Maddon insists the best is yet to come for Jake Arrieta.

If that's the case, then it's scary to think how good the 29-year-old right-hander might become.

Arrieta pitched seven innings of two-hit ball and combined with two relievers on a four-hit shutout while first baseman Anthony Rizzo tied a career high with four hits as the Cubs beat the Pittsburgh Pirates 5-0 on Tuesday night for their sixth straight win.

Arrieta (12-6) allowed just two singles -- by center fielder Andrew McCutchen in the fourth inning and left fielder Starling Marte in the seventh -- while striking out five and walking three.

"He's an interesting pitcher," Maddon said. "I will say it again, he's not there yet. He's still learning how to utilize all his weapons. Once he does that, he's going to be able to be more efficient with his pitchers and go deeper into games.

"There's another level of Arrieta."

Right-handed relievers Tommy Hunter and Justin Grimm completed the shutout with one inning each as the Cubs (58-47) matched their longest winning streak of the season from May 11-16 and moved 11 games over .500 for the first time since the end of the 2008 season.

In his last nine starts, Arrieta is 6-1 with a 1.38 ERA. The right-hander improved to 5-1 in his career against the Pirates.

"They have a good lineup from top to bottom and they have good at-bats," Arrieta said. "A lot of pitches I threw, even the walks, were pretty quality. I was just being conscious of not giving them much to hit. I had that mindset of not giving in."

Rizzo doubled and scored in both the third and fifth innings as he led the Cubs' 14-hit attack with the fourth four-hit game of his career.

"We haven't been contenders for a long time and it's fun to see how everything is starting to come together for us," Rizzo said.

Cubs left fielder Chris Denorfia had three hits while shortstop Starlin Castro doubled in Rizzo twice and right fielder Jorge Soler drove in the first and last runs with a bases-loaded walk in the first inning off J.A. Happ (4-7) and a single in the sixth.

Chicago drew within three games of the National League wild card-leading Pirates (61-44), who remained 5 1/2 games behind the St. Louis Cardinals in the NL Central.

Happ had a rocky Pirates' debut, giving up four runs and eight hits in 4 1/3 innings with two walks and six strikeouts. The left-hander was acquired from Seattle last Friday in a trade and was 0-1 with a 9.00 ERA in his last four starts with the Mariners.

"I was anxious to get out there," Happ said. "I put us in a tough spot but I'm going to be better and I'm starting to look forward to the next one."

Marte had two hits.

Happ ran into trouble early and forced home the first run with the bases-loaded walk to Soler.

The Cubs made it 3-0 in the third when Castro hit an RBI double and scored on catcher David Ross' RBI single.

Rizzo doubled again with one out in the fifth inning then scored when Castro greeted reliever Jared Hughes with his second double.

That was more than enough cushion for Arrieta.

"I think the one point in time we'd seen 90 pitches and only had one hit and we were just rolling into the seventh inning," Pirates manager Clint Hurdle said. "We couldn't find a way to connect any dots on him."

NOTES: Cubs LHP Tsuyoshi Wada was activated from the 15-day disabled list Tuesday and optioned him to Triple-A Iowa. Wada had been out since June 23 with left deltoid inflammation and was on a rehab assignment with Iowa. ... Rookie C Kyle Schwarber was not in the starting lineup for a second straight day as the Cubs again faced a left-handed starter. ... Chicago CF Dexter Fowler went 0-for-5 but drew a walk to run his streak of getting on base to 18 games in a row. ... RHP Dan Haren (7-7, 3.42 ERA) will make his Cubs' debut Wednesday night when he faces Pittsburgh LHP Jeff Locke (6-6, 4.21 ERA) in the finale of a series shortened to two games by rain. Haren was acquired from the Miami Marlins on Friday in a trade.