Top seed Samantha Stosur survived a scare against Alize Cornet before beating the Frenchwoman 7-6, 7-5 to reach the Kremlin Cup quarter-finals on Thursday.
On the last match on centre court, 43rd-ranked Cornet served for the first set but failed to take her chances against the 2011 US Open winner, losing a tiebreak 8-6.
Cornet also led 5-4 in the second, and after wasting three set points ran out of gas as Australian Stosur secured a hard-earned victory just before midnight.
The men's top seed, Alexandr Dolgopolov of Ukraine, was not so lucky as he went down to 116th-ranked Edouard Roger-Vasselin, who saved four match points in a 6-3, 5-7, 7-6 win.
The French qualifier forced a tiebreak after saving two match points on his serve in the 12th game of the third set.
In the tiebreak Roger-Vasselin recovered from 6-4 down to win the last four points, sealing the win when Dolgopolov's forehand went long.
"(Dolgopolov) just didn't play well on those match points. He looked a bit tense out there," the 28-year-old, who reached his third quarter-final of the year, told reporters.
Roger-Vasselin now takes on giant Croatian Ivo Karlovic, who edged out Russian Alex Bogomolov Jr 7-6, 6-4 after serving 16 aces.
Seventh seed Carlos Berlocq of Argentina was also knocked out while Italy's second seed Andreas Seppi and fourth-seeded Brazilian Thomaz Bellucci advanced.
Seppi beat Dutchman Igor Sijsling 7-5, 6-2 and Bellucci eliminated Italian Flavio Cipolla 6-4, 6-1.
In the women's event, fourth-seeded Serbian Ana Ivanovic booked her spot in the last eight with a routine 6-1, 6-4 win over Russian qualifier Valeria Solovieva.
"I think she looked nervous, especially in the first set, because it was probably her first big match," said former world number one Ivanovic, who next meets 158th-ranked compatriot Vesna Dolonc.
Moscow-born Dolonc, who took Serbian citizenship last year, crushed Romanian Simona Halep 6-1, 6-0 to reach her first quarter-final on the WTA Tour this year.