The 30-year-old was gifted point after point as Ostapenko’s aggressive game backfired in a blaze of unforced errors.
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It will be a repeat of the 2016 final, which saw the American come out on top with a 7-5 6-3 victory
The top seed won 3-6 6-4 6-1 in just over two hours to hoist the Coupe Suzanne Lenglen in front of a packed centre court.
Williams has been in full training for just a month, though, and the 23-time Grand Slam winner has decided to delay her return to the big stage until she regains her sharpness.
However Williams will be encouraged by her overall play in her first match since winning the 2017 Australian Open while four months pregnant with her daughter Alexis Olympia.
Williams has not played since winning the Australian Open in January, while she was six weeks pregnant, but will return to court to play French Open champion Jelena Ostapenko on Saturday.
Both players had lost their opening ties on Sunday, and were desperate for a victory to keep their future fate in the eight-woman event in their own hands
Ostapenko was only a few weeks old when Williams made her Wimbledon debut 20 years ago.
In only her 18th match in one of the four majors, the 20-year-old Latvian fought back in audacious fashion from a set and 3-0 down to blaze past the experienced Romanian