PHOENIX (AP) Left-handed batter Alek Thomas took a confident swing at a slider and hit a crucial three-run homer off one of baseball's top left-handed pitchers.

It was another indication that these young Arizona Diamondbacks are growing up fast, getting better by the game in a pressure-packed September playoff chase.

Lourdes Gurriel Jr. and Thomas both hit three-run homers, rookie Brandon Pfaadt delivered 5 1/3 quality innings and Arizona beat the Chicago Cubs 6-4 on Friday night.

Thomas came into the game hitting just .121 against lefties, but connected in the sixth inning off Cubs ace Justin Steele, delivering a two-out homer that just cleared the right-field wall and pushed the Diamondbacks ahead 6-0.

Steele (16-4) is one of the leaders in the National League Cy Young race.

“Pretty sweet feeling, being able to help the team and increase the lead,” Thomas said.

The two clubs are in the middle of a six-team dogfight for three National League wild-card spots. Arizona moved into a tie with the Cincinnati Reds for the final spot, while the Cubs are in the second position, 1 1/2 games in front of the D-backs and Reds.

Gurriel's homer to deep left in the first gave Arizona an early 3-0 lead. It was his 24th homer of the season.

“We talk about stepping up in the most critical parts of the game, in the inning, and having the right at-bat,” Arizona manager Torey Lovullo said. “Even though it was early, it made a statement for me. The dugout was pretty excited.”

The Cubs trailed 6-0 heading to the ninth before Ian Happ hit a solo homer and Seiya Suzuki added a two-run shot off reliever Kyle Nelson. The Diamondbacks then brought in closer Paul Sewald, who immediately gave up a no-doubt solo homer to Christopher Morel.

“Hopefully, we can carry that into tomorrow,” Cubs manager David Ross said. “We didn't get a lot going early on.”

Sewald quickly recovered, retiring the next two batters.

Steele gave up six earned runs on seven hits and two walks over six innings. He had been on a roll before Friday's start, giving up just one run over his previous 21 innings.

Pfaadt (2-8) delivered one of the best outings of his short big league career, striking out six while giving up four hits and a walk.

“For me, it just looked like he was in total control throughout the course of his outing,” Lovullo said. “The bullpen took it from there. I know what we saw in the last inning, but that stuff happens. Paul came in and slammed the door.”

Catcher Gabriel Moreno added a career-high four hits, including a double.

The Diamondbacks took three out of four games from the Cubs in Chicago last week.

STROMAN RETURNS

Cubs All-Star Marcus Stroman was activated from the 15-day injured list and delivered two scoreless innings of relief on Friday.

The 32-year-old right-hander has been out for about six weeks with right hip inflammation. He has a 10-8 record and a 3.85 ERA in 23 starts this season, but manager David Ross said he’ll be in the bullpen for the time being.

“Marcus has a proven track record and getting him back was an important step,” Ross said. “He’s on a journey to try and build back up, but right now, he’ll help us out in the bullpen. Then we’ll read and react to see how everything goes these last couple weeks.”

TRAINER'S ROOM

To make room for Stroman, the Cubs optioned RHP Daniel Palencia to Triple-A Iowa.

Lovullo said RHP Drey Jameson (elbow) had some discomfort in his arm during his throwing program. Lovullo added that the team is still evaluating what the next steps will be. Jameson has been on the injured list since July.

UP NEXT

The series continues Saturday. The D-backs will start RHP Zach Davies (2-5, 6.81 ERA), while the Cubs counter with RHP Kyle Hendricks (6-7, 3.71).

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