Back in action after sitting out last week in Toronto with a sore back, the world number two showed no hint of rust as she breezed through the opening set in 28 minutes.
But it was American King, helped along by a sparse but vocal centre court crowd, who took control early in the second set by breaking the grunting Azarenka at the first opportunity on way to a 3-0 lead.
Azarenka, dripping with sweat on humid morning in Ohio, dug deep to pull level at 3-3 before forcing the set to a tiebreak, which she again rallied to win 8-6.
"The second set, the beginning of the second set wasn't very good for me," the second seeded Belarusian told reporters. "There were quite a few unforced errors and just really fast mistakes which didn't happen in the first set.
"She was already in the game playing at a much higher level and really going for her shots. So I had to stay with her and take my opportunities, and I did.
"There was just a little bit few inconsistencies, but first matches can be tricky."
Azarenka kicked off a busy day at the Western and Southern Open, which brings together the top players from the ATP and WTA Tours in a joint event that for many will be their final tune-up to the U.S. Open.
Frenchwoman Alize Cornet provided the upset of the morning session by knocking off 15th seed and former world number one and French Open winner Ana Ivanovic of Serbia 2-6 7-6 (8) 6-4.
Belgian 13th seed Kirsten Flipkens also made an early exit falling 3-6 6-1 6-3 to Russia's Elena Vesnina.
(Editing by Frank Pingue)
