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.
Top-seeded Simona Halep had to fight hard to advance to the second round of the Prague Open in beating unseeded Polana Hercog 6-1, 1-6, 7-6 (3) yesterday.
It was the first competitive match for Halep since she won the Dubai Championships in February for her 20th career WTA title.