Fox, 30, got off to a great start in yesterday's opening round, with two birdies in the first nine holes, before unravelling with four bogies and a double bogey down the stretch to sit 80th overnight.
He had no such problems today, birdying the seventh hole that had cost him so dearly a day earlier, along with the third, seventh, ninth and 10th holes.
But his improvement can actually be traced back to that first-round collapse, where he learnt from taking a different approach late in the day.
"I really struggled off the tee most of yesterday and missed everything right, not bad, but bad enough to not have too many shots at the greens.
"I had a little bit of grip change on the eighth tee - my second to last hole - and hit one dead straight down there and a pretty decent one down nine, even though it ended up in the rough. I took that into today and it seemed to work pretty well."
This is Fox's third Major appearance - he played the last two British Open Championships - and his first in the US.
"I'd like to [play more in the US], but if every golf course was this difficult, I think you'd burn out pretty quickly. I've really enjoyed it so far."
New Zealand number one Danny Lee has carded a seven-over 78 in his second round to currently be tied for 137th at 12-over.
