The No. 1-ranked American was shown in footage on Australia's Channel 7 network coughing into a towel and blowing her nose before she abandoned the practice session on Friday.
She later said in a statement released by the WTA: "I had a false start (this morning). I wasn't feeling really well. I've been sick with a cold all week, and I got better, then I got worse the morning."
She was feeling well enough to return to the practice court in the afternoon.
Williams will be aiming for her 19th Grand Slam singles title when she takes on Sharapova on Saturday.