The Briton defeated the 26-year-old in the quarter-finals of the Brisbane International back in January of this year.
"Technically he's very sound," BBC Sport quotes Murray as saying. "He's a good player.
"He hits the ball very flat. He had a very good match with Novak [Djokovic] in Montreal. He's had some good wins this week, as well."
Istomin has reached the fourth round of a Grand Slam only once before.