Former world number one Serena Williams recovered from a set and three break points down to beat compatriot Bernarda Pera in her first match in six months.
The American, who has 23 Grand Slam singles titles, was in deep trouble when 4-6 and 0-40 down at 4-4 in the second set at the Top Seed Open in Kentucky.
But she recovered to prevail 4-6 6-4 6-1 in two hours and 15 minutes.
"I knew I could play a little bit better if I hung in," Williams said.
Williams will play the winner of the match between her sister Venus and Victoria Azarenka - another former world number one - in the next round.