The misfire set up Adrian Beltre for a two-out, walk-off single in the bottom of the ninth that gave the Rangers a hard-fought 4-3 win -- not to mention a three-game sweep -- over the Red Sox.
Things had started so well for Mortensen, who was pitching in a more pressurized situation than normal on a day the Red Sox were once again without closer Andrew Bailey (right biceps injury).
Mortensen came on in a 3-3 tie and promptly struck out the first two batters he faced. Elvis Andrus gave the Rangers life with a single up the middle.
And with Lance Berkman at the plate, Mortensen buried the first pitch. The ball bounced off catcher Jarrod Saltalamacchia, and then too far away to make a play on Andrus, who was on his way to second.
Once Berkman looked at the next pitch to make it a 2-0 count, Red Sox manager John Farrell called for an intentional walk. Four pitches later, Beltre lined one to right for a single and Andrus scored easily from second.
"Yeah, it's frustrating," Mortensen said. "You come out and get two outs pretty quickly, and the next guy jumps on the first pitch and gets a base hit. You say, 'OK, let's get this next guy,' and then you throw one in the dirt, it gets away, he goes to second, and it kind of changes how you're going about it in the inning. It just kind of changed the complexion of the inning."
It marked the first time this season the 20-11 Red Sox have been swept, and perhaps it's fitting that it came against the team that is now tied with Boston for the best record in the Majors.
"It's three games. We're not going to get overwhelmed by winning three," said Rangers manager Ron Washington. "I'll say it again -- we can play baseball. On any given night, our best game can compare to anyone's."
The Red Sox are now 12-28 against the Rangers dating back to the beginning of the 2009 season.
The good news is that the Sox headed back to Boston after the game, set to open a seven-game homestand that starts Monday night against the Twins.
"We'll be fine," said Red Sox lefty Jon Lester, who took a no-decision. "We've got a bunch of guys in this clubhouse -- this isn't their first go-round. Guys have been through the winning streaks and losing streaks and everything in between. I wouldn't imagine it's going to be a big thing for us.
"We played a good clean game today. They had some really good at-bats and squared some balls up at the wrong times. Sometimes you have to tip your hat to the other team when you have games like this and move on to tomorrow."
The Red Sox had an early 3-0 lead on home runs by David Ortiz and David Ross, but Rangers ace Yu Darvish was nasty for the rest of the day.
"Going up against a very good starter in Darvish today, we got to him early then he settled down and was dominant through the seven innings of work that he pitched, but we continued to battle," Farrell said. "We created some opportunities late. Just, unfortunately, we didn't get that one key hit."
While there was a lot of buildup about the starting-pitching matchup, Lester and Darvish wound up dueling to a virtual standoff.
Over six innings, Lester allowed five hits and three runs, walking three and striking out seven.
"I felt good," Lester said. "I felt like I had pretty good stuff. I made two mistakes. Against a team like that in this ballpark, you can't do that."
Darvish lasted seven innings, giving up four hits and three runs. He also tied a career high with 14 strikeouts.
"He's capable of throwing breaking pitches hard and soft for a strike," Ortiz said. "He had no clue about where his fastball was going, but he can throw his off-speed pitches for strikes a lot. He's got good command of his slider and two different breaking balls. The fastball was bouncing all over the place, so I wasn't worried about that."
Starving for offense in the first two games of the series, the Red Sox got an early jolt from Ortiz, who ripped a two-run homer to right against Darvish with two outs in the first. That extended Ortiz's career-long hitting streak to 25 games, dating back to last season.
"I just reacted to a pitch that I saw," Ortiz said. "It wasn't in -- it was on the plate."
In the second, the Red Sox got another big swing, this one from Ross. The backup catcher smashed a solo shot over the foul pole in left, and into the club level. Ross became just the fourth opponent in the history of Rangers Ballpark to hit one into the club seats in left.
"Not that many people are as good as I am," quipped Ross. "It was, 'close my eyes and swing hard.' That's my approach."
It marked just the second time in his career Darvish allowed two home runs in one game.
After that, Darvish was almost untouchable. Early on, so was Lester.
The lefty did give up a solo homer to Mitch Moreland in the third, slimming Boston's lead to 3-1.
Lester navigated out of jams in the fourth and fifth, but he was not so fortunate in the sixth. With Beltre on first, Lester left a fastball out over the middle of the plate to Nelson Cruz, and the outfielder roped it for a game-tying, two-run homer to center.
"The guys did a good job early on of giving me that lead, and I've got to do a better job of minimizing damage, and I didn't do that in the sixth," said Lester.