In eight innings, Lester (11-4, 2.81 ERA) gave up nine hits on four runs (three earned) while striking out six and walking three.
"That was probably the best I have felt physically in a long time, and I didn't take advantage of it," Lester said. "Whether that be because of a few days off, or just feeling too strong or overthrowing the ball, I made some adjustments, got the ball down in the zone and they just got hits with guys on base, and that is the name of the game.
"You can give up all the solo home runs you want, when you get those two- or three-run innings, that is what kills you."
The Rangers got on the board in the fourth with a two-out rally, with a single by Josh Hamilton and a double by Nelson Cruz. After Cruz's line-drive double to center scored Hamilton from first, the Rangers right fielder moved to third on the throw home and scored the go-ahead run when third baseman Adrian Beltre couldn't handle catcher Dusty Brown's throw.
"We got two out and then the ball falls in front of left field and Lester throws a breaking ball to Cruz," manager Terry Francona said. "It was the right pitch, wrong location and it scored the one run and we don't convert the end of the play that is another run."
After a Michael Young strikeout in the fifth, with Julio Borbon at third and Elvis Andrus at first thanks to a double and a single, Andrus stole second and before Brown could tag Borbon, he swiped home.
Borbon is the first Rangers player to steal home since Ricky Ledee on Sept. 23, 2001, against the Angels.
In the eighth, after a leadoff double by Andrus and a walk by Young, Ian Kinsler singled to left, scoring Andrus for a 4-1 lead. Francona came out to argue the call with home-plate umpire Gary Darling, and replays showed the tag being applied just before Andrus slid into home.
"He just said he was safe," Francona said. "His explanation didn't seem to correspond to what I saw. That happens."
However, Andrus agreed with call.
"I was safe. He tagged me on the shoulder," Andrus said. "I'm not a midget. My legs are long enough."
While the Rangers continued to tack on runs against Lester, the same could not be said against Texas pitcher C.J. Wilson. In 6 2/3 innings, Wilson struck out a career-high 10 and gave up only one run on three hits.
The Sox got on the board when Kevin Youkilis led off the second with a single to left, followed by an RBI double to left-center field by Beltre.
"He was getting ahead a lot," center fielder Mike Cameron said. "He was kind of tough. Strike one, strike two right away and a couple of times, I was 0-2 before I even got a chance to get in the box score. It was a constant battle and when he got a chance to get out of a couple of jams, he just pumped up a little bit and threw it by people and we popped it up or something. He was efficient. The guy was efficient."
Boston threatened in the sixth and seventh innings, putting runners on first and second and two outs. However, Beltre struck out swinging to end the sixth and Marco Scutaro popped out to center field to end the seventh.
Cameron led off the ninth with a home run into the Green Monster seats to cut the deficit to 4-2. With Bill Hall on base after an error by Kinsler, Scutaro lined out to Andrus at short to end the game.
With the loss, the Red Sox fell to 3-8 since July 4. The series loss at home is the club's first since it dropped two of three against the Yankees in May.
"We're trying to win series," Rangers manager Ron Washington said. "It's hard to come up here and do what we did this weekend. I don't care how many guys they are missing, those guys know how to play baseball and still have some guys in the lineup that can do some damage. But we never backed down. We still played our game."
The Red Sox travel to Oakland to begin a 10-game road trip Monday that will also take them to Seattle for four games and Anaheim for three.
"Every game is important this time of year," Lester said. "You have to play good baseball. I think we need to grind some things out like we have done all year. We have had injuries and that is no excuse. We will keep grinding away and trying to win.
"That is one thing about our team, we always fight and will never give up, even today we had the tying run at the plate in the bottom of the ninth. That is all you can ask for."
Quinn Roberts is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.