PHOENIX -- It was an uncharacteristically bad night for Justin Steele, and the timing of the Cubs' Cy Young Award candidate’s rough outing came at the wrong time. 

The 28-year-old, who has the second-lowest ERA in the National League, gave up a season-high six earned runs in a series-opening 6-4 loss to the D-backs at Chase Field.

The Cubs have now lost six of their last eight games, and with the D-backs’ victory, have also lost the tiebreaker to Arizona.

• Games remaining (14): at AZ (2), vs. PIT (3), vs. COL (3), at ATL (3), at MIL (3).
• Standings update: The Cubs (78-70) are 5 1/2 games behind the Brewers (81-64) for first place in the NL Central. The Cubs and Brewers have split the season series so far, so the tiebreaker has not yet been determined. Chicago sits in the second Wild Card spot, trailing the Phillies (80-67) by 2 1/2 games, and holds a 1 1/2-game lead over the third-seeded Reds (77-72). The D-backs (77-72) have the same record as Cincinnati, but the Reds hold that tiebreaker. The Marlins and Giants are also in the hunt. The Cubs have won the tiebreaker over San Francisco, but lost it to Philadelphia, Cincinnati, Miami and -- after Friday’s loss -- Arizona.

“I actually felt really good coming in today,” Steele said. “I felt like the off-day really helped me yesterday ... I felt really good. I had a lot of 94-95 stuff. I made a few mistakes, and they did what they were supposed to do.”

After his rough first frame in which Steele surrendered a first-pitch three-run homer to Lourdes Gurriel Jr., the lefty settled in a bit, allowing just three hits from innings two through five, before things unraveled again in the sixth. Steele issued a free pass to Walker and Gabriel Moreno singled to right field, which prompted pitching coach Tommy Hottovy to pay a visit. 

On Steele's next pitch, Illinois native Alek Thomas slugged his ninth home run of the season to stake Arizona to a 6-0 lead. Steele finished the frame, wrapping his night with six innings, allowing seven hits, six earned runs and striking out five. 

It was the first time Steele has allowed two home runs in a game since July 16.

“Just execute the pitches a little bit better,” Steele said. “The two homers I gave up just weren't executed as I would like to. I was just trying to get a strike [against Gurriel] to start the count 0-1. Like I said, he did exactly what he was supposed to do with it and hit it out of the ballpark. And then the same thing with Alek Thomas.”

It was a complete turnaround from Steele’s start against the D-backs last Saturday, when he held them to one run over seven innings. Arizona certainly had a different game plan, but manager Torey Lovullo made sure to keep it a secret.

"I don't want to talk about it,” Lovullo said. “We might face the guy again. But I think we recognized certain pitches and certain counts and we didn't miss them."

D-backs starter Brandon Pfaadt held the Cubs scoreless over 5 1/3 innings, allowing four hits and one walk. Last week, the Cubs scored four earned runs on seven hits against Pfaadt at Wrigley Field.

“I thought he filled up the strike zone,” Cubs manager David Ross said. “It's a lot easier to pitch with a three-run lead, coming right at you, throwing strikes, getting the secondary for strikes. Then he threw some nice 3-2 breaking balls. But I didn't think we got a real good sense of what he was trying to do to us at any time.”

Marcus Stroman, who was activated off the injured list before Friday’s game, took over and pitched two innings, allowing one hit and striking out the side.

After being quiet for most of the first two hours of the contest, the Cubs’ offense woke up in the ninth inning.

Ian Happ, Seiya Suzuki and Christopher Morel each homered, with Suzuki’s plating two runs, but it was too late for a comeback as Paul Sewald retired Nick Madrigal on a popout to end the game.

“Hopefully the carryover is positive for the last inning,” said Happ, who hit his 18th home run of the season. “There were some good swings. I think that's a good team, and [Pfaadt] pitched well. There's going to be days like that, but the team's pretty professional and we'll come out tomorrow and swing the bat.”