After winning 17 games out of 20, the Royals have dropped three in a row. This was the opener of a rare five-game series, brought about by a rainout earlier in the year. The teams will play twice on Friday.
Kansas City scored its lone run in the first inning, but Fielder immediately wiped out that deficit by hitting his 18th home run of the season.
Sanchez (11-7) protected the lead by going 7 1/3 innings. He allowed seven hits and struck out five. Joaquin Benoit walked a batter in the ninth on the way to his 16th save.
Hoping to make up ground on the first-place Tigers in the American League Central during the long series, Kansas City fell 8 1/2 games behind.
Jeremy Guthrie (12-9) absorbed the loss, allowing all four runs on 13 hits in seven innings.