Isner defeated Ryan Sweeting 7-6 (7-1), 7-6 (7-4) to set up a semifinal meeting with Feliciano Lopez.
Lopez had a 6-4, 2-2 lead when Carlos Berlocq retired with a injury.
Russell, who defeated No. 1 seed Mardy Fish in the third round, advanced with a 6-1, 5-7, 6-3 victory over Ryan Harrison.
Monaco fought off Kevin Anderson 7-6 (7-4), 7-5.
Notable deaths of 2014 [PHOTOS]
UPI horse racing weekend preview