Home favourite Aslan Karatsev had to save four set points on his way to a 6-4, 6-3 win over Gilles Simon at the Kremlin Cup in Moscow on Friday.
Karatsev, who has not dropped a set all week at the event, broke Simon five times in the match and will face Karen Khachanov next.
Khachanov beat Aussie John Millman 7-5, 7-6 (7-5) in lengthy see-saw battle that lasted two hours and 26 minutes.
Khachanov took a 4-3 lead only for Millman to break back to draw level to 4-4.
The Russian then grabbed a second break to take the set.
In the second set Khachanov opened up a 5-1 lead before Millman pulled it back to 3-5, but Khachanov then closed out the tie with a break on match point.
