World number one Serena Williams was forced to regroup but launched her bid for a ninth Miami Open crown by defeating US compatriot Christina McHale 6-3, 5-7, 6-2.
Three-time defending champion Williams had the match on her racquet before having to withstand a fightback from her 23-year-old rival and taking two hours and seven minutes to reach the third round.
"She just started playing better, the kind of tennis she's never played before," said Williams on Thursday.
"I made a few mistakes and then I had to pull myself back together. I feel like I was just playing, but it's all right. I'm back now and I'm going."
Top seed Williams, who watched tournaments at the combined WTA and ATP hard court event as a child, will next face Zarina Diyas of Kazakhstan, who ousted Australian Daria Gavrilova 7-5, 6-3.
Williams fired 13 aces and 36 winners with 43 unforced errors while McHale responded with seven aces and 26 winners against 31 unforced errors.
Asked if she took McHale too lightly, Williams replied, "I don't underestimate anyone."
Williams, whose back-to-back finals losses at the Australian Open and Indian Wells were her first since 2004, served for the match in the 10th game of the second set but netted a backhand to surrender a break.
McHale held and won the first three points of the 12th game off Williams' serve. The 34-year-old pulled back two but swatted a forehand wide to force a third set.
"She definitely started fighting back," Williams said. "And she's clearly capable of playing great tennis."
McHale denied Williams on three break points to hold the first game of the final set but Williams broke in the third and smashed a backhand service return winner to break again in the seventh game.
Williams had to fight off two break points in the final game, saving one with her 12th ace and the other with a service winner, and she blasted her 13th ace on match point to finally advance.
After answering a few questions, Williams told reporters, "I just really don't want to be here. I feel like I've been doing press everyday. That's enough," and ended her session.