Cook, England's most prolific test run scorer, has made 62 runs in four innings as the tourists have gone 2-0 down in the five-match series and some former players turned pundits have suggested he might be about to call time on his career.
"I haven't made any decision on anything. All my focus is on this game, the biggest of our lives coming up," Cook said in Perth.
Cook captained England on their last Ashes tour Down Under when spinner Graeme Swann retired mid-series with the tourists 3-0 down and headed for a 5-0 whitewash.
Former Australian paceman Mitchell Johnson and England batsman Kevin Pietersen have suggested that Cook had the look of a man who might be ready to hang up his bat.
"For the people who are saying that, they've had no contact time with me," Cook, who is 33 later this month, said. "They wouldn't know the extra nets I've been doing behind closed doors.
"I was with (my batting coach) yesterday for an hour-and-a-half in the morning, desperate to keep working on my game.
"That's probably not a guy who's given in. To be honest with you, I have no idea (when I'll retire). And I've said that since I gave up the captaincy."
The opener said retirement might not end up being his own choice if he continued to fail to make runs and said becoming the first England player to play 150 tests was "quite special", especially as the last 147 have been consecutive.
Cook has enjoyed an Ashes triumph in Australia in 2010-11 as well as the 2013-14 debacle and said the current party would not fail for want of trying.
"I will say this about this England side, I've never seen a side a work as hard as this side," he said.
"Win, lose or draw, whether we play well or we play rubbish, the effort from the guys is unbelievable. There's a group of men in there, 16 or 17 of them, desperate to do well."
First Published: December 12, 2017, 1:33 PM IST