Cincinnati took a 2-0 lead through two innings behind starter Homer Bailey. Facing Dodgers lefty Hyun-Jin Ryu, leadoff man Billy Hamilton bunted for a single in the first inning. After a Brandon Phillips' single, Joey Votto hit a potential double-play ball to first base. Miguel Rojas' errant throw into the seats allowed Hamilton to score.
In the Reds' second inning, Todd Frazier led off with a single to center field against Ryu. Frazier scored on Hamilton's RBI single to right field.
Bailey allowed one walk and struck out two over three scoreless and hitless innings while throwing 33 pitches.
"I got through three innings, that was kind of important," Bailey said. "I was trying to build up the endurance and stuff. I was working on a couple of breaking balls. Overall, it wasn't too bad. I really didn't go too deep in many counts. They were pretty aggressive out there."
Los Angeles pounced on the next pitcher, Jeff Francis, who was in his second inning of work in the fifth. Adrian Gonzalez and Juan Uribe started a rally with back-to-back singles and Joc Pederson walked. Next came Guerrero, who hit a 1-1 Francis pitch a long way over the left-field fence for the grand slam. It was the rookie's first extra-base hit with the Dodgers.
"Home runs always help psychologically," Guerrero said. "I just want to make good contact, that helps my mindset. I feel happy, my first home run in the U.S. at this level."
It was the third grand slam Reds pitchers have allowed this spring.
There was one out in the sixth when Gonzalez crushed a solo homer to right field against Reds reliever Sam LeCure. It became a four-run frame when a two-out mistake prolonged the inning. Left fielder Skip Schumaker misplayed Tim Federowicz's fly ball near the fence. It bounced on the warning track and over the fence for a two-run ground-rule double. A Rojas RBI single to center field made it a six-run game. Only one run against LeCure was earned.
Up next: The Dodgers get their first game with the expanded instant-replay rules on Thursday when they play the Angels in Tempe at 12:05 p.m. PT. Dan Haren will make his second start of the spring. Yasiel Puig, who exited with a sore back in the fifth inning on Wednesday, said he should be back in the lineup.
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, and follow him on Twitter @m_sheldon. This story was not subject to the approval of Major League Baseball or its clubs.