Arroyo shook off a shaky start, Todd Frazier hit a two-run home run and Ryan Ludwick and Jay Bruce added solo shots as the Reds beat the Chicago Cubs 7-3 in the opener of a four-game weekend series on Friday night.
Cincinnati, coming off a loss to the New York Mets on Thursday that snapped a five-game winning streak, began the day leading the NL Central by six games and improved to a major league-best 25-9 since the All-Star break.
"Usually, we've had big slides in the second half, and that's taken the wind out of our sails," Arroyo said. "This year, we feel like the ball's always rolling in your direction."
Manager Dusty Baker liked knowing the Reds would at least pick up ground on one of their two closest pursuers with second-place Pittsburgh playing at third-place St. Louis.
"Any time you win, it's good, but with the Pirates playing the Cardinals, you know you're going to pick up ground on somebody," he said.
Arroyo (9-7) has been a big part of the surge, improving to 5-1 in his last six starts. He gave up nine hits and three runs with three walks, three strikeouts and a wild pitch in 6 1/3 innings.
The Reds had six extra-base hits in five innings against streaky former teammate Travis Wood (4-9), who has lost his last six decisions after winning four straight starts.
Luis Valbuena led off the second with his fourth homer of the season and first since June 29 against Houston. His 418-foot fly into the right-center field seats gave Chicago a 1-0 lead.
Chicago capitalized on another Reds fielding misplay to take a 3-1 lead in the fourth. After Castro led off with a single and Valbuena flied out, Brett Jackson hit a line drive to second baseman Brandon Phillips that got away from him after he appeared to be distracted by the passing Castro.
Castro went to third on what was ruled a single and scored on a wild pitch to Steve Clevenger, who then walked. Wood drove Jackson in from second with a single to center for the pitcher's first RBI of the season.
The Reds then got five runs and six hits in the bottom half of the inning, starting with Ludwick's 23rd homer of the season, a 425-foot drive into the left field seats on Wood's first pitch of the inning. Wood hit Bruce with a pitch, and Frazier gave the Reds a 4-3 lead by hitting a 2-2 pitch 419 feet into the left field seats for his 16th homer of the season.
"I couldn't really tell you what was going on," Frazier said. "He throws that cutter a lot. I was sitting on it. That was a big five-run inning for us."
"It didn't look good for a while, but we answered back when they scored," Baker said. "We were on (Wood) pretty good in that inning."
Wood allowed more runs and hits in that inning than the five hits and one run he allowed in seven innings against Cincinnati last Sunday in Chicago.
"The doors fell off," he said. "We took a game plan out there, and I didn't make adjustments soon enough."
Friday's Wood obviously wasn't the same as last Sunday's, Cubs manager Dale Sveum said.
"He probably spent everything he had in that last outing," Sveum said. "His velocity was down, and his pitches were flat. He went into a gunfight with a knife."
Bruce hit Wood's 0-1 pitch 411 feet into the right-field seats with one out in the fifth for his 25th homer of the season.
The streaky Wood, traded with two other players by Cincinnati to Chicago for pitcher Sean Marshall during the offseason, lasted five innings, getting roughed up for nine hits and seven runs -- six earned -- with three strikeouts.
RHP Todd Redmond is expected to be recalled from Triple-A Louisville on Saturday to start the second game for the Reds in their split doubleheader against the Cubs. Redmond, acquired from Atlanta for INF Paul Janish on July 14, has been recalled three times by two teams this season, but he's still waiting to make his major league debut. ... The Cubs are expected to recall LHP Brooks Raley from Triple-A Iowa to face Redmond. ... Hall of Fame second baseman Joe Morgan flipped the coin to determine who would serve first for Friday's Western & Southern Open tennis quarterfinal match between Serena Williams and Angelique Kerber in suburban Cincinnati.