The Englishman lost his PGA Tour card after being hampered by a foot injury over the second half of the season, but he hit form in Shanghai as he returned an encouraging seven-birdie card.
The 34-year-old, who is just three shots behind leader Fredrik Jacobson heading into the weekend, says that he has had to alter his swing to take the pressure off his troublesome right foot.
But everything appeared to click in round two as he made a significant move up the leaderboard.
"That was one of the best rounds of golf I've had in a long time," said Casey, who has shown little form since lifting the Volvo Golf Champions in Bahrain in January.
"The golf I played is the golf I remember playing. I've been trying to keep my right foot down as long as possible - it doesn't hurt as much and it makes a massive difference.
"I can suddenly control where the golf ball is going."
Casey stormed to the turn in 32 and came home in 34 although will still feel that he left some shots out there. He ran up a bogey six at the 14th and missed a short birdie at the par five last.
China has been a happy hunting ground for the world number 22.
Two of his 11 European Tour wins have come there while, in this event, he's racked up five top 11 finishes in his six appearances
Casey is spearheading a strong English challenge so far this week.
He shares fifth place with American Bo Van Pelt, while it's just a shot back to world number two Lee Westwood in tied seventh.
A further shot in arrears - and just five off the pace in tied 11th - are the trio of Justin Rose, Ian Poulter and Simon Dyson.
It's been a bit of a different story for 20-year-old Tom Lewis, who slumped to a 78 in his first round in a WGC event on Thursday. However, he made a decent recovery on Friday, shooting a one-under 71 to finish at five-over.