With the temperature at 96 degrees at the opening pitch, Hernandez fought both the heat and the red-hot Sox, who have the best record in the Majors in July at 14-3.
Seattle, on the flip side, has gone 4-14 this month to fall out of the American League West chase at 43-56. The 13-game losing streak is the second-longest in franchise history, one short of the record 14 set in 1992.
Hernandez hasn't won since June 24, going 0-3 with a pair of no-decisions in his last five starts. He's 8-9 on the season, with a 3.47 ERA.
Rookie Mike Carp ripped a three-run home run in the eighth to make things closer for the Mariners, but they again came up short on a night they managed 11 hits.
The reigning AL Cy Young Award winner gave up 11 hits in 6 1/3 innings, but wiggled out of four different jams with double-play grounders and was trailing, 2-1, before the wheels came off in the seventh.
A two-run bases-loaded single by Adrian Gonzalez knocked Hernandez out of the game and reliever Jeff Gray couldn't squelch the damage as Kevin Youkalis doubled in two more. David Ortiz followed with a run-scoring single to make it 7-1.
Hernandez kept things close to that point, thanks in large part to the four double plays, including a gem by shortstop Brendan Ryan behind the bag at second on a hard smash by Ortiz, with runners on first and second in the third inning.
Ryan did it again in the fifth, turning a bases-loaded grounder by Youkalis into an inning-ending double play to keep the score at 2-1, with Hernandez laboring after 86 pitches in the heat.
But once again the Mariners struggled offensively until Carp's second career home run and first of this year. Although they managed eight hits in seven innings off Boston starter John Lackey, seven were singles and only one led to a run -- an RBI base hit by rookie second baseman Dustin Ackley in the first.
Ichiro Suzuki went 2-for-5 with a double in the seventh. A night after manager Eric Wedge issued the club a postgame, closed-door address, Ichiro came out aggressively in the first inning with a leadoff single, followed quickly by stolen bases of second and third.
He then scored Ackley's single to left, giving Hernandez a brief 1-0 lead.
But the Red Sox responded with a run in the bottom of the first on an RBI single by Youkalis and took the lead on a home run in the third by Jacoby Ellsbury, then broke things open with their five-run seventh.
Hernandez came into the game with the lowest ERA of any pitcher with at least five career starts at Fenway Park, at 1.49. But after Friday's loss, he's 3-1 with a 2.53 ERA in six Fenway outings.