Anthony Rizzo had two hits and three RBIs, including a solo homer into the Cubs bullpen in right in the fourth off Ariel Pena (2-1).
Arrieta (22-6) allowed two singles in his final tuneup before the postseason. His ERA dropped to 1.77.
His next start will likely come against Pittsburgh in the NL wild card game. Entering Friday, the Pirates held a two-game lead on the Cubs in the race to host the game with three games left in the regular season.
The Pirates were tied with the Reds in extra innings when the Cubs game ended.
