Xtina did Etta proud.

Christina Aguilera sang at Saturday's memorial service at an L.A. church for the late Etta James -- crooning the legendary blues' singer's classic "At Last" for hundreds of mourners. (Stevie Wonder sang with the church choir, while Rev. Al Sharpton delivered the eulogy.)

In photos of Aguilera's incredible tribute to James -- who passed away Jan. 20 -- fans noticed odd streaks running down her legs.

"She sang her heart out. It was a memorial service for her idol and she was nervous," an insider explains to Us of the performance. (Aguilera, 31, has been covering "At Last" for years and called James "a true gift to music and a woman of great spirit and courage" on her website after James' death."

"It was her proudest moment," the source explains. "The sweat caused her spray tan to streak."