It was like chalk and cheese for the 2004 runner-up, who was trailing 5-1 when the match was interrupted on Thursday because of bad light.
Dementieva was simply another player when the match started again in clear sunshine despite the Ukrainian pocketing the first set on Dementieva's seventh double fault.
However, the 24-year-old went on to grab all games in the second, then struggled to wrap up the contest after two hours and one minute of play on her second match point.
Dementieva, who will now meet Israel's 31th seed Shahar Peer, faces a possible fourth-round clash against former world number one Martina Hingis of Switzerland.
REUTERS DH HS2031