how lucky can Istomin be! he had bye in the first round of qualifications, entering on the main draw as a LL eventually and he plays a tired Grigor now!
Dimi is a young stud. He played epic tennis for like 5 hours against Nadal. I think his fitness won't be a problem. Only played 11 sets last weeks and was mostly in control.
However he might be mentally exhausted after that epic Aussie semi final, emotional victory at home, and then a really annoying match against Misha Zverev.
Istomin has a serious chance, I think he takes it in 2.