Top-seeded Simona Halep had to fight hard to advance to the second round of the Prague Open.
The Williams sisters will face each other in a second-round showdown on Thursday.
Williams was five points from falling to the American left-hander in the second set but suddenly showcased her fighting spirit and pulled away to set up a second-round showdown with older sister Venus, a 6-3 6-2 winner over Victoria Azarenka. The Top Seed Open, the first WTA Tour tournament in the U.S. since the COVID-19 outbreak halted play, provided a taste of what is to come for the 38-year-old American as she bids for a record-equalling 24th Grand Slam title in New York, with empty, silent stands taking the place of roaring crowds. Venus, who fell in the first round at the Australian Open, Acapulco, and Monterrey earlier this year, cruised through her clash of former world number ones as she turned aside the one break point she faced during an 80-minute win over Azarenka.