DEFENDING champion Ian Rayson faces one of his most difficult challenges in the Puma Hong Kong Championship at Tai Mei Tuk on Saturday in the form of two leading international triathletes - Simon Knowles and Rick Kiddle. Knowles has been one of Australia's most consistent competitors over the Olympic distance of 1.5-kilometre swim, 40-kilometre bike and 10-kilometre run since he turned professional in 1991. The 25-year-old made the top four in all 11 races on the Australian circuit in his second season, including five wins - results which earned him the Australian Triathlete of the Year award for 1993. ''I've never raced Simon before but he should beat me in all three disciplines,'' said Rayson, the Hong Kong number one who is also considered one of the best in Asia. ''Simon is a minute ahead of me in the swim, so he should be leading all the way. The only thing that can stop him is a puncture.'' Another top challenger for the Hong Kong race is Kiddle, a member of the British team for the past six years. Rayson and 31-year-old Kiddle were in the British team together before the former was posted to Hong Kong while serving in the air force. Rayson has left the services and is now a coach of the triathlon association. ''It should be an interesting race between Rick and I for second place,'' Rayson said. ''Our times are quite similar. Rick was about 30 seconds faster at last year's World Championships in Manchester, but we were racing in a different category and his race was on first. ''But I reckon I should have the edge in Saturday's race because I know the conditions better.'' Rayson tipped the Chinese - Quo Weidong, Zhang Jian and Zhang Xuefeng - as underdogs because there is no record on their past achievements and he also rates Malaysia's Razzani very highly.