The Kyrgyzstani lightweight followed up on triumphs over Brad Riddell, Bobby Green, Renato Moicano, Marc Diakiese and Alex White and should now leap up the lightweight rankings.
The scorecards were relatively even at the end of round four, though Fiziev had the slight edge, but he made sure in the final round delivering third knock down of dos Anjos' career.
For the first time in his entire MMA career, Fiziev made it to a fourth round and dos Anjos sensed the opportunity to take advantage catching Fiziev with a flying knee and a left hand. But Fizie came roaring back in the fifth, a jab and a big left hand from the southpaw knocked do Anjos off his feet, and some follow-up punches on the ground finished the job.
Fiziev now has his sights set on number five ranked Justin Gaethje, who is the former interim lightweight champion and two-time title challenger.
"I want a fight with Gaethje," Fiziev said after beating dos Anjos. "We need to know who is the best for you and for me. If you're ready, if you're no scared, if you don't take s**t, let's go."
"He's dangerous. He's also a big challenge for me. He has a lot of chance for knocking me out, and I have a lot of chance for knocking him out.
"That's a good fight, I think, for fans also."