Fox carded a one-over 71 - featuring two birdies and three bogeys - to climb three places and sit eight strokes off the lead in New York.
His best previous finish in 15 appearances at Major tournaments was his 16th at the 2019 British Open.
Four-time major winner Brooks Koepka led the field with a 4-under-par 66 for the second straight day to take control.
Koepka carded five birdies and just one bogey on a day when only nine players broke par. The LIV Golf star hammered home a 47-foot birdie putt at No. 17 in order to get to 6-under 204.
Koepka overtook Corey Conners of Canada and Viktor Hovland of Norway, who shared the lead with Scottie Scheffler entering the day.
Late mistakes held Conners (70) and Hovland (70) at five-under for the championship. Bryson DeChambeau also shot 70 to remain in fourth at three-under.
Scheffler bogeyed four of his first seven holes and didn't get a birdie to drop until No. 14 en route to a 73. He dropped to two-under, tied for fifth with Justin Rose of England (69).
Nearly 0.9 inches of rain fell at Oak Hill, mostly between 8 a.m. and 4 p.m. local time, making everyone's rounds a slog but particularly affecting those who played early. Englishman Tommy Fleetwood's 68 was the low round for most of the day until Koepka polished off his 66.
Koepka led the Masters through three rounds after grinding his way through sloppy weather conditions on Friday and Saturday, similar to what players faced at Oak Hill.
But he yielded to eventual champion Jon Rahm of Spain, and on Sunday he will try not to repeat what he described earlier in the week as a "choke."
Conners' second birdie of the day, a 9-footer at the par-3 15th, made him the first player to reach 7 under this week. But he gave it back at the par-four 16th in ugly fashion.
Conners' attempt to get out of a fairway bunker at No. 16 barely left the ground, hitting the face of the bunker in front of him and plugging. He was given a free drop but his third shot went from rough to rough. The ensuing double-bogey 6 ruined what would have been the only bogey-free round of the day.
Hovland could have tied Koepka at six-under but landed in a greenside bunker at No. 18 and blasting out way past the hole, leading to a finishing bogey.
PGA professional Michael Block made three birdies on the back nine to shoot his third consecutive even-par 70 and was tied for eighth with Justin Suh (73). He is expected to play with Northern Ireland's Rory McIlroy (seventh place, 1 under) on Sunday.
Block, the head pro at Arroyo Trabuco Golf Club in Mission Viejo, Calif., could become the first club pro to finish in the top 20 at a PGA Championship since 1990. The last to finish inside the top 10 at the PGA was 50 years ago -- when a 61-year-old Sam Snead, retired from the tour, competed as a club professional.
- Additional reporting RNZ
