
Williams, who said she was not feeling 100%, saved her best tennis for the biggest moments, breaking Petkovic for a sixth time in the final game to make up for a dismal second set.
Asked in an on-court interview to describe the encounter, the 38-year-old former world number one was almost at a loss for words.
“I don’t know honestly. I’m still trying to figure it out,” she said to laughs from the crowd on a sunny and windy day in the California desert.
“Every day is not your best day but that doesn’t mean the heart and desire aren’t there.”
The win sets up a mouthwatering second-round clash with third seed Petra Kvitova, who holds a 4-2 edge in their previous meetings.