Ferrer, a finalist at this year’s French Open, broke serve four times as he completed the win in 62 minutes.
Isner will face France’s Jeremy Chardy, who came from a set down to beat Florian Mayer 4-6, 7-5, 6-3 in one hour and 45 minutes.
Valencia Open results
7-Fabio Fognini (Italy) beat Marcel Granollers (Spain) 6-3 6-2
Dmitry Tursunov (Russia) beat Roberto Bautista (Spain) 6-2 7-6(3)
1-David Ferrer (Spain) beat Gael Monfils (France) 6-3 6-2
Philipp Kohlschreiber (Germany) beat 2-Tommy Haas (Germany) 3-6 6-3 6-3
Jarkko Nieminen (Finland) beat Benoit Paire (France) 6-3 6-3
Jeremy Chardy (France) beat Florian Mayer (Germany) 4-6 7-5 6-3