Despite 16 aces, Dimitrov failed to convert any of his three break-point chances.
Janowicz will next play Benoit Paire, who defeated Peter Gojowczyk 6-2, 6-4.
Third-seeded Tomas Berdych also advanced, beating Bernard Tomic 7-6 (4), 6-2. Berdych will next face Feliciano Lopez in the quarterfinals. Lopez defeated Jeremy Chardy 6-3, 3-6, 6-2.