The top seeded Spaniard sealed victory in one hour and 40 minutes after losing just one point on serve in the deciding set.
It was the world number three's 55th win of the season and if he now beats compatriot Nicolas Almagro, he will book a place in his seventh final of the year and second on the bounce, following Stockholm.
"It was a very tough match," said Ferrer, who has won the title in Valencia three times before. "[Janowicz] is a really good player."
Almago secured his place in the semis with a 7-6(6) 6-2 win over Fabio Fognini, and was full of praise for his Italian opponent.
"Fabio is having an extraordinary season in hands of someone very special to me, Jose Perlas. It was with him that I reached the top 10," said Almagro.
"During the match there were a lot of ups and downs and I knew that if I kept the right mindset the opportunity would come."
In the other semi-final, Dmitry Tursunov will play fellow Russian Mikhail Youzhny.
Valencia Open results
3-Nicolas Almagro (Spain) beat 7-Fabio Fognini (Italy) 7-6(6) 6-2
1-David Ferrer (Spain) beat 5-Jerzy Janowicz (Poland) 6-4 4-6 6-0
Dmitry Tursunov (Russia) beat Jeremy Chardy (France) 6-3 6-3
Mikhail Youzhny (Russia) beat Jarkko Nieminen (Finland) 7-5 6-7(4) 6-4