HOUSTON -- Colby Rasmus was the unquestioned star of the show after breathing life into a flaccid Astros' offense, but the bottom of the Houston order produced in manners commensurate to Rasmus' heroics.

Rasmus belted his fifth career grand slam, lifting the Houston Astros to an 8-3 win over the Boston Red Sox on Saturday at Minute Maid Park.

Rasmus crushed a 2-2, two-out pitch from Red Sox right-hander Clay Buchholz (0-2) off the facing of the second deck in right field to snap a 1-1 tie in the fifth inning and ostensibly end the Astros' four-game skid. Prior to his sixth homer, Rasmus was 3-for-26 against Buchholz.

"Front door two-seamer started up on the plate and ended up middle," Buchholz said. "It's a really good pitch whenever you're able to throw it where you want to but the ball was moving a lot today. For the most part felt like I did a pretty good job with executing the pitches I was throwing at times. Me and Colby faced each other a lot. He got me today."

Houston (6-12) rallied in support of right-hander Mike Fiers, who allowed one run during a treacherous first inning but steadied himself to pitch effectively one out into the sixth.

Fiers (2-1) surrendered two runs on five hits and two walks with six strikeouts. After allowing the first three batters to reach in the first inning, a stretch that included an RBI single from David Ortiz, Fiers settled in and retired 15 of 16 batters before second baseman Dustin Pedroia opened the sixth inning with a leadoff double to left field.

"My fastball command wasn't there early on," Fiers said. "I kind of had to go to my changeup early and I was throwing it for strikes and expanding the zone with two strikes. I was able to get out of some jams early."

Pedroia eventually scored when Brock Holt greeted reliever Will Harris with a booming sacrifice fly to left, but Harris snuffed that rally and Boston (8-9) managed just one additional run via a two-out Josh Rutledge pinch-hit double in the eighth inning off right-hander Ken Giles.

Buchholz was unbeaten in four career starts against the Astros, a ledger that included two complete games. And, excluding a hiccup in the bottom of the second in which he allowed Houston to pull even when Jason Castro recorded an RBI groundout, Buchholz cruised early.

"He was throwing the ball good," Red Sox manager John Farrell said. "The one inning today bit him."

With two outs in the fifth Buchholz allowed a single to George Springer before plunking Carlos Correa with a pitch to load the bases. Buchholz got ahead of Rasmus with two nasty off-speed pitches but then opted for a fastball that Rasmus promptly slugged for a 5-1 lead.

"I wasn't necessarily surprised (by the pitch selection), but he had a good changeup," said Rasmus, who was 2-for-5 with five RBIs. "He had good stuff. His changeup was probably as good as I've seen and he's always got a pretty good one. So, yeah, in that spot I just kind of stepped back and said, 'Yeah, if he throws that changeup then I swing at it early again.'

"I'm not going to take a chance on missing that heater and he threw it where I was looking and thankfully I was able to connect with it."

While Jose Altuve finished 3-for-5 with a pair of doubles batting leadoff for Houston, Marwin Gonzalez and Luis Valbuena went a combined 4-for-6 with two walks hitting seventh and eighth, respectively.

NOTES: Red Sox RHP Carson Smith worked an inning at extended spring training and should begin an injury rehab assignment with Triple-A Pawtucket on Tuesday. Smith, acquired via trade from Seattle during the offseason, was placed on the 15-day disabled list on April 3 with a right flexor muscle strain. ... RHP Eduardo Rodriguez threw 75 pitches in extended spring training and will be sent to Double-A Portland to begin his injury rehab assignment on Thursday. Rodriguez, placed on the 15-day DL with a right patella subluxation on April 3, is scheduled to make at least two rehab starts before being considered for recall. ... The Astros continue to be plagued by ineffective early-inning pitching, with the two first-inning runs surrendered by RHP Collin McHugh on Friday night bumping the total to first-inning runs allowed by the Astros to 23, roughly 27 percent of their total runs allowed this season. ... Red Sox SS Xander Bogaerts was not in the starting lineup after being hit by a pitch on the right wrist in the ninth inning on Friday night. Bogaerts was hopeful that he might be available defensively in the interim until the soreness subsides.