Fish had little trouble notching a 6-2, 6-4 win over unseeded German Mischa Zverev during the match at the Delray Beach Stadium & Tennis Center. Fish is seeking to repeat as Delray champ after beating Evgeny Korolev of Kazakhstan in last year's final.
Fish, who also was the tourney's runner-up in 2003, will face France's Jeremy Chardy in Friday's quarterfinals after Chardy, the No. 4 seed, downed Colombian Santiago Giraldo 7-5, 4-6, 7-5.
Blake, the No. 7 seed, downed Robin Haase of the Netherlands 6-3 6-1 and will face Croat Ivo Karlovic, winner of a tough 7-6 (10-8), 7-6 (8-6) match over German Daniel Brands.
No. 3 Benjamin Becker of Germany gained the quarters with an easy win over American Wayne Odesnik, who pulled out with an ankle injury. Becker will face Finnish veteran Jarkko Nieminen in the quarters.
Also Thursday, Latvian Ernests Gulbis dominated Russian Teimuraz Gabashvili 6-1, 6-4 and will meet Argentine Leonardo Mayer in the quarters.