Top-seeded Dementieva beat New Zealander Marina Erakovic 6-2, 6-3, while second-seeded Wozniaki beat Jill Craybas 6-4, 7-5.
Dementieva showed improved form after struggling in her first round match to beat Erakovic, who was supported by the crowd.
"It was a very emotional match,'' she said. "I think we had a good fight in the first set and I played a little better than yesterday and this is what I need, just to improve and keep playing better tennis in every round.''
The third, fourth, sixth, seventh and eighth seeds fell to unseeded opponents on Wednesday, after all the tournament's eight seeds had safely reached the second round.
Third-seeded Anabel Medina Garrigues lost 6-3, 6-3 to Edina Gallovits and No. 4 Aleksandra Wozniak lost 7-5, 6-2 to qualifier Ayumi Morita.
Sixth-seeded Nicole Vaidisova was beaten 6-4, 6-3 by Elena Vesnina, No. 7 Anastasia Pavlyuchenkova lost 6-2, 6-3 to Aravane Rezai and No. 8 Carla Suarez Navarro lost 6-2, 6-4 to Anee Keothavong.
The only other seeded player to progress was Israeli No. 5 Shahar Peer, who shrugged off threatened protests over Israel's military action in Gaza to beat Barbora Zahlavova 6-3, 4-6, 6-2 in Wednesday's only singles match to go to three sets.
Peer was surrounded by extra security as she played her second round match after an Auckland-based protest group called on her to withdraw from the tournament as part of an international boycott of Israel.
Peer rejected the call, telling reporters later Wednesday that she could not be held responsible for the politics of the Middle East.
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