"At one stage, I was looking directly at the lady. It was not a good moment.
"I felt very, very sad. I really hope she's ok."
Tomic was relieved to hear that was the case.
"Tennis is tennis, sport is sport. But if someone passes out or on dies on a tennis court, it can be very, very bad," he said.
"It was something I've never experienced in a match ... even the referee I spoke to said it never happened before."
Tomic next faces Simone Bolelli, meaning he will be expected to advance to the third round with minimal fuss.
"I've never played him before in my career. I'll try to suss as much as I can," he said.
Tomic is seeded 16th - never before has he earned such a great chance to progress to an elusive Open quarter-final.
But any thoughts the divisive 23-year-old had of steamrolling Istomin were extinguished in the opening set, when he saved three break points to push it to a tiebreaker.
"He was playing so well in that first set ... he was serving so well," Tomic said.
"I've played Denis many times. Last time he beat me so I knew it was going to be difficult.
"He's beaten very good players in the past. I knew he would come out like that."
The Uzbek won the opening four points of the breaker, taking the first set with one of his 23 aces.
Tomic twice broke the world No.59's serve in the second set then showed impressive poise in the third, saving five break points at 5-4 to serve out the set.
Tomic, who insisted he pulled out of last week's Sydney International because he was sick, had mixed feelings about the match.
"I wasn't feeling that great the last few days. I was trying to minimise time on court and do the right things," he said.
"I feel physically pretty well (now).
"I played up and down ...I'm very happy I won."
- AAP
