Knox was initially seventh reserve for the £5.5million HSBC Champions at Sheshan International, but the first four were drafted in to complete the 78-man field before England’s Tyrrell Hatton replaced the injured George Coetzee.
Brandt Snedeker withdrew last week to hand Ian Poulter a place in the tournament and when JB Holmes was also forced out, Knox found himself competing for a first European Tour title and the first prize of £900,000.
The 30-year-old from Inverness carded an opening 67 and added a flawless 65 in tougher, blustery conditions on Friday to lie two shots behind American Kevin Kisner, who added a 66 to his opening 64 and has not dropped a shot in 36 holes.
Knox found out he would be making his WGC debut while playing in the CIMB Classic in Malaysia last Friday and praised his wife Andrea for making the necessary arrangements - and also stepping in as a temporary caddie.
“My wife was a superstar and she filled out all the forms for us,” said Knox, who made his Open debut at St Andrews in July after McIlroy was forced to withdraw due to the ankle injury suffered playing football with friends.
“While we were playing on Friday she took all the stuff to the Chinese embassy in Kuala Lumpur and had it processed. Then we had to wait around till Monday at 2pm to get them back and they gave us our passports.
“We flew here on Tuesday morning but my caddie didn’t have proof of accommodation so he was a day behind us. So when we got here my wife caddied for me. We got a stand bag from the pro here and chucked the clubs in and played the quickest practise round ever and she complained heavily for the last nine holes!”
Asked if that would earn Andrea a share of his winnings this week, Knox joked: “Don’t worry, there was a nice mall across the street where she took care of herself!”
McIlroy, who recovered from a bout of food poisoning to card a 68 on Thursday, had come within inches of a hole-in-one on the 12th, but bogeyed the 13th after missing the green and was furious with another pulled iron shot on the short 16th, slamming his club into the bag and then barely bothering to line up a par putt from four feet, which he duly missed.
Things looked set to get even worse on the 18th when McIlroy found water with his approach and had to let two more balls roll into the hazard as he took a penalty drop on a steep slope short of the green.
However, the four-time major winner pitched to four feet to save par and signed for a level-par 72 which left him four under overall.