The 41-year-old Venus, who has seven Grand Slam titles to her name, said she would miss competing at her "favorite slam" and wished all the other players good luck
World number one Ashleigh Barty will meet ninth seed Petra Kvitova in the quarters, while eighth seed Belinda Bencic is up against Spaniard Paula Badosa, ranked 62 in the world.
Unfortunately I am not feeling 100% in my lower back, and I have made the tough decision to withdraw from the 2021 Dubai Duty Free Tennis Championships.
"I have great memories from winning my 20th WTA title after some really tough matches in Dubai last year so I'm disappointed that I won't be able to defend my title."
World number two Halep is no slouch around the court, and the pair shared 52 rallies of five shots or more during a heavyweight contest on Rod Laver Arena.
"I think 1926, the summer of 1926 I think was the last time I felt that," the 39-year-old smiled.