Nix, whose contract was purchased from Triple-A Colorado Springs, started the season with the Rockies, but struggled and was sent down. Since then, the second baseman has hit .299 with 14 home runs and 43 RBIs in the Pacific Coast League.
"I was just playing [in Triple-A] and doing the best I can down there," Nix said. "I just got in a groove, it wasn't like I changed a lot, but I got locked in there for awhile."
Nix flew out of Sacramento on Tuesday morning, was in Milwaukee by the afternoon, and was in manager Clint Hurdle's starting lineup for the night's game.
"That's where you want to be," Nix said. "You try to be the same every day, go out there with the same approach."
In order to make room for Nix on the 25-man roster, outfielder Seth Smith was optioned to Triple-A. To make room on the 40-man roster, lefty Josh Newman was designated for assignment.
The move to bring up Nix adds more versatility to Hurdle's infield and also insurance for a wrist injury to Jeff Baker. Baker jammed his wrist sliding into second base on a steal during Monday's 4-3 win over the Brewers.
He sat out of pregame activities -- including batting practice -- before Tuesday's game, but X-rays of the wrist were negative and Baker said that the rest was more of a precaution than anything else.
"I don't think this is going to be too bad," Baker said. "I really don't."
Dave Fultz is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.