The Twins scored five runs in the first inning off Lewis, and Jim Thome later drove in four runs with a pair of home runs off him. That gives Thome 584 home runs, allowing him to pass Mark McGwire for ninth all-time.
"They just put it to us today," Rangers manager Ron Washington said.
"I didn't make pitches when I needed to today," Lewis said. "I left pitches in the middle of the plate and that's what happens."
The Rangers have lost six of their last nine games, reducing their lead to eight games over the Athletics in the American League West. The Rangers have also lost 10 of their last 14 games on the road.
Texas has also lost its first five games at Target Field. The only place worse for the Rangers is AT&T Park in San Francisco. The Rangers have lost all nine games they have played there.
This was the second-most runs Lewis has given up in a Major League start. He allowed 10 runs in a start for Oakland on May 22, 2007, against the White Sox, and Thome had a three-run home run in that game.
Lewis had been pitching well early in the losing streak while being victimized by a lack of run support. But the pendulum is swinging the other way. He has an 8.46 ERA in his last four starts, having allowed 23 runs (21 earned) on 31 hits and six walks over 22 1/3 innings.
"He just wasn't at his best today," Washington said. "He threw a lot of pitches up in the zone, something he usually doesn't do. You just keep going out there trying to make adjustments, that's all you can do."
Lewis is 0-7 with a 5.07 ERA in his last nine starts, dropping his record to 9-12 with a 3.56 ERA on the season.
"It is what it is," Lewis said. "You just continue to grind it out. It's a situation where you go and compete and try to keep your team in the ballgame. I feel great. There is nothing physically wrong with me. I feel like I did on Opening Day."
Lewis started his afternoon by throwing a strike to Denard Span. Then he threw four straight balls to issue a leadoff walk and the afternoon went downhill from there.
Orlando Hudson hit his next pitch for a single. Lewis got Michael Cuddyer to hit a grounder to first baseman Mitch Moreland but the only play was to first base. Lewis then walked Thome to load the bases and retired Delmon Young on a pop to second.
That brought up J.J. Hardy with two outs. Lewis had a chance to get out of the inning without giving up a run and was able to get ahead 1-and-2 in the count.
Lewis then missed with a fastball and Hardy fouled off a slider low and away. Another fastball missed to run the count full and then Hardy fouled away a fastball on the outside corner. Finally Lewis threw a slider that Hardy blasted to deep left-center for a ground-rule double.
"I was just trying to see the ball out of his hand," Hardy said. "Honestly, I was kind of looking for his slider. I was late on a couple fastballs and then 3-2 he threw me another fastball and then fouled it off. He threw the slider I was looking for, he hung it a little bit and I was able to put a good swing on it."
It was the key at-bat of the inning and ultimately the game.
"I thought I threw a couple of pitches inside and didn't get the call," Lewis said. "The 3-2 slider ... maybe that's his bat speed. He had a good at-bat, grinded it out and hit it where he needed to."
Lewis then hit Jason Repko with a pitch, reloading the bases, and Matt Tolbert followed with a triple into the right-field corner to bring home three more runs.
"It all happened with two outs," Washington said. "They scored five runs with two outs. That's something I wish wouldn't have happened."
Thome made it 6-0 with a home run off Lewis in the third, then added a three-run shot off him in the fourth. That was the last batter Lewis faced on the afternoon.