Noren, Karlsson and Van Zyl, who is from South Africa, each turned in a 5-under-par 67 and lead a group of four players, which includes another Swede in Niklas Lemke, by one stroke.
Nine more golfers are at 3-under-par. That includes 22-year pro Robert Karlsson, who is no relation to the co-leader Robert Karlsson -- a 22-year-old amateur.
The amateur Karlsson could well have owned the outright lead but had a double bogey at the par-3, 164-yard 17th hole. However, he had a birdie at No. 18. He was 4-under after eight holes, after a string of three birdies on Nos. 6-8.
Noren also had a double bogey that threatened a strong start. Playing the back nine first, he had five birdies in his first six holes. After four pars, he had a 6 at the par-4 second hole and a bogey at No. 4. He got back into the lead with a birdie at the fifth hole and closed with birdies at 8 and 9.
Van Zyl's round was less dramatic, mixing six birdies and one bogey for his share of the lead.
Wisconsin business offering 'therapeutic cuddling' forced to close
Mena Suvari shares her delightfully awkward Christmas card photo