Rain pelted down over Roland Garros for the second day running and Williams capitalised on the 90-minute window to overcome rising French prospect Alize Cornet 6-4, 6-3.
Williams had expected to face Cornet on Sunday but instead ended up spending the day twiddling her thumbs in the locker room as drizzle washed away most of the opening day's schedule.
After enduring a further three-hour delay on Monday, the American took out her frustrations on her 17-year-old opponent.
Playing in front of a smattering of fans, Williams survived four games going against the serve and took the first set with a stinging crosscourt winner.
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