Arrieta pitched eight shutout innings and ended his three-game losing streak as the Cubs defeated the Oakland A's 4-0 at the Oakland Coliseum.
Arrieta (13-5) allowed three hits, struck out four and walked one, winning for the first time since June 27 against Cincinnati. Arrieta was 0-3 with a 4.88 ERA in his previous five starts after going 12-2 with a 2.10 ERA in his first 16.
Arrieta said he is "really close" to being back to where he wants to be.
"Emphasis on sinking the ball down, middle down in the strike zone and forcing action early on," he said. "Not worrying about strikeouts. Those will come in big situations."
The Cubs won their sixth straight game and improved to 68-41, the best record in baseball. They moved 27 games above .500, matching their season high, and will go for a sweep of the three-game series against Oakland on Sunday afternoon.
After dropping nine of 10 games from Jun 30 to July 9, the Cubs have won 16 of their past 22 games.
"It's a good streak that we've been on," Cubs second baseman Ben Zobrist said. "We've played some good games. We're pitching a lot better, we're hitting a lot better. Just having better quality at-bats than we were before the (All-Star) break. It just seemed like we really needed the rest of the All-Star break to get back to the type of team that we are."
Zobrist went 2-for-5 and drove in two runs. Addison Russell had two hits in four at-bats, drove in a run and scored once. Jorge Soler had an RBI double.
Oakland right-hander Sonny Gray (5-11) allowed two runs and five hits but was taken out after just five innings and 72 pitches because of what the A's called right extensor muscle soreness on the top his forearm. He struck out two, walked none and retired the final seven batters he faced.
"Hopefully it's minor," A's manager Bob Melvin said. "He was fighting to go back. He was throwing the ball pretty good today."
Gray said he felt soreness in the fourth inning and again in the fifth.
"Hopefully it will feel OK the rest of the day today and then coming in here tomorrow," Gray said. "That's going to be the big test."
The A's fell a season-high 14 games below .500 at 48-62, losing for the seventh time in their past eight games.
The Cubs took a 2-0 lead in the top of the third when Willson Contreras and Dexter Fowler opened the inning with back-to-back singles and scored on Zobrist's two-out single to center.
The Cubs extended their lead to 4-0 in the sixth, scoring twice against reliever Liam Hendriks. Anthony Rizzo led off with a double and scored on Russell's single with one out. Soler doubled with two outs, bringing Russell home.
Arrieta gave up a two-out double to Yonder Alonso in the second inning, a single to Marcus Semien in fifth and a single to Ryon Healy in the eighth. He hit a batter in the third and walked one in the fourth but didn't let a runner get past second base.
"Walked a guy, hit a guy, got locked back in," Arrieta said. "Good mechanically. Good finish. And threw a lot of strikes. That's a good game plan for me."
NOTES: Athletics rookie LHP Dillon Overton was optioned to Triple-A Nashville and LHP Patrick Schuster was recalled from the Sounds to take his spot on the 25-man roster. Overton is 1-3 with a 10.97 ERA in five starts for Oakland and has allowed 11 home runs in 21 1/3 innings. Schuster, a rookie, had a 15.43 ERA in four relief appearances in his first stint with the A's. ... RHP John Lackey will start for the Cubs on Tuesday at Wrigley Field when they open a two-game series against the Angels. RHP Jason Hammel, who was placed on the bereavement list Wednesday, will start in the series finale against the Angels. ... Willson Contreras' primary position is catcher, but he started in left field for the 18th time this season for the Cubs. He has started 17 games at catcher and two at first base. ... The A's have not named a starter for their game Tuesday against Baltimore, but manager Bob Melvin said RHP Zach Neal and Triple-A RHP Jharel Cotton are two options. Cotton was one of three minor league right-handers acquired from the Dodgers for RF Josh Reddick and LHP Rich Hillon Monday. Neal has been pitching out of the bullpen for Oakland. ... A's LHP Sean Doolittle (strained left shoulder) threw a bullpen session for the first time since going on the disabled list June 26. He threw 25 pitches.