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.
Williams' last competitive action was in the Fed Cup tie against Latvia in February and it showed in the early stages as she surrendered the first set and offered up break points in the first game of the second.
However, after repeating her escape from 0-40 down in the ninth game, the momentum swung decisively in Williams favour.
Williams, who plans to play both the US Open and September's rearranged French Open in a season badly disrupted by the coronavirus pandemic, is one of the star names at the inaugural event in Kentucky.
The tournament, the women's tour's first since the death in police custody of George Floyd sparked worldwide protests calling for racial justice, is being played on courts bearing the 'Black Lives Matter' slogan.