Janowicz made the most of momentary lapses in concentration by Dimitrov, who followed top seed Roger Federer, beaten by Tommy Haas on Wednesday, out of the Wimbledon warmup event.
"This was a very important match for me. I have to fight for more wins. I am happy to be back where I was two years ago after I was injured, struggling with health," said Janowicz, 26, who will now face Frenchman Benoit Paire in the last eight.
Bulgarian Dimitrov, the world number 12, has now reached only one quarter-final since winning his last of two titles this year back in February.
A sensational backhand down the line - the shot of the match - gave Janowicz the minibreak he needed in the first set tiebreak. Another whipping forehand earned Janowicz, a former world number 14 who has been plagued by injuries in recent seasons, his first two break points on Dimitrov's serve.
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The Bulgarian then sent one of his forehands long to allow his opponent to serve out the match.
Czech Tomas Berdych, at three the highest seed still in the tournament, also booked a last eight spot with a 7-6(4) 6-2 win over Australian Bernard Tomic. He will next face Feliciano Lopez of Spain.
(Reporting by Karolos Grohmann; editing by Mark Heinrich)
