Ortiz, who also scored three runs, is 11-for-20 (.550) with five RBIs since returning last weekend. He missed 71 of last year's final 72 games and most of the first three weeks this season because of a heel injury.

"I've been working, just like to stay short and quick to the ball, not trying to do too much," Ortiz said. "And I always tell you guys, I'm old enough to know what it takes for me to be what I want to be."

Boston has won 10 of its last 13 and taken the first eight series openers of a season for the first time in team history.

Clay Buchholz (5-0) allowed two runs and six hits in 7 2/3 innings with 10 strikeouts and two walks. His ERA rose from 0.90 to 1.19.

The 28-year old right-hander became the first Boston pitcher to win his opening five starts since Josh Beckett in 2007 -- when the Red Sox went on to win the World Series.

"Last couple of years I've been a slow starter, so it feels good to be out there and have a little bit of confidence early on, and that helps a lot," he said. "Guys are playing well behind me. Things are going right."

Pitching one year and four days after his perfect game for the Chicago White Sox against Seattle, Philip Humber (0-5) gave up seven runs, 10 hits and three walks in 4 2/3 innings. Humber, 2/3 with and 8.36 ERA in five career starts against the Red Sox, has allowed 12 first-inning runs in his last two starts, including four against Boston.

"I'm just not making good pitches. It's not fun to go out there and get hit around like that," he said. "Like I said, if I had an answer, I would have done something different. Give credit to their hitters, they've got good hitters and didn't miss the pitches they should hit. It was pathetic. I didn't give our team a chance to win."

Houston dropped to 7-15, the Astros' worst start since opening with the identical record in 1975, according to STATS.

"It's definitely not fun," manager Bo Porter said. "It's hard when you fall behind early in a game against one of the better pitchers in our league. You're on the road, and you're fighting an up hill battle."

Ortiz and Dustin Pedroia hit run-scoring singles in the first, when Mike Carp added a two-out RBI double and scored on Jarrod Saltalamacchia's single.

Houston closed to 4-2 on run-scoring singles by Fernando Martinez in the second and Jason Castro in the third.

Ortiz homered into the center-field bleachers in the third, his first since July 13 against Tampa Bay's Jeremy Hellickson and his first at Fenway Park since June 27 off Toronto's Jesse Chavez.

Saltalamacchia hit a run-scoring single in the fifth and scored on a double by Will Middlebrooks, Humber's final batter.

Game notes
Boston is 11-0 when scoring first. ... Farrell expects RHP John Lackey, on the 15-day DL with a strained right biceps, will start Sunday. ... Red Sox OF Shane Victorino, who left Wednesday's game with a stiff lower back, didn't play. ... The Astros are on their second trip to Fenway. They were swept in a three-game series in June of 2003. ... Boston recalled C Ryan Lavarnway from Triple-A Pawtucket , filling the spot that opened when RHP Alfredo Aceves was optioned to Triple-A following Wednesday's game. Lavarnway is expected to go back to the minors when Lackey returns Sunday. ... The Red Sox gave out nearly 6,200 free tickets to fans that had tickets to Tuesday's game, played in a steady drizzle with a game-time temperature of 42. ... Boston closer Joel Hanrahan, on the 15-day DL with a strained right hamstring, is expected to begin a rehab assignment with Pawtucket on Friday.