Fields is nursing a sore left wrist after getting hit by a pitch in the first inning against Texas on Sunday.
Nix, who came off the 15-day disabled list on May 1 after battling a strained right quad, has never started at third base in his brief Major League career.
So Nix made sure to get to Kauffman Stadium early on Monday afternoon to get some extra practice at the position.
Nix said he felt comfortable at the hot corner, but the extra fielding on Monday helped.
"You know, I haven't played it a whole lot," said Nix, who started at second base against the Rangers on Sunday and went 2-for-3 with an RBI in his first action of the season.
White Sox manager Ozzie Guillen said he liked what he saw from Nix at the plate.
"We got to put the best guys out there," Guillen said. "I think Nix, we got to figure out how he plays third base."
Rustin Dodd is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.