Unseeded Hingis, champion on the clay here in 1999, had a surprisingly easy time of it against the Russian on another day of brilliant sunshine and near-perfect conditions at the Steffi Graf stadium.
Dementieva had won the pair's last three matches in straight sets but only one of those was since January, when the 25-year-old Hingis made her return from a three-year retirement.
"I needed a great deal of self-confidence out there today," Hingis said.
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Dementieva, one of six Russians in the last 16 here, had a great chance to get back into the match with Hingis on 0-40 in the fourth game of the second set. But the Swiss held her nerve, and her serve, for 3-1.
In the quarters Hingis will face either world number one Mauresmo of France, who was in imperious form on Wednesday, or Russia's Anna Chakvetadze. They play later on Thursday.
Mauresmo beat Hingis 6-2 6-2 in the semi-finals in Doha in March, their only meeting since Hingis's return.