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.
Serena Excited For Aussie Final
Serena Williams, who has been battling a cold, talks about her semifinal win over Madison Keys and looks ahead to the final at the Australian Open where she will face Maria Sharapova.
Aussie Open: Women's Final
Maria Sharapova and Serena Williams look ahead to the Australian Open final.