Williams, the Australian Open champion and former top-ranked women's player, shut out Czech Zuzana Ondraskova 6-0, 6-0. Ondraskova joined the draw Tuesday when Russian Elena Bovina pulled out of her scheduled bout with Williams.
Up next for the eight-time major titlist Williams will be Ukrainian Julia Vakulenko.
Dementieva, a two-time major runner-up, defeated two-time Grand Slam champion Mauresmo of France 6-2, 7-5. Mauresmo was unseeded this week, mostly due to inactivity this season.