PROMISING Billy Kwok Sze-wai outshone his more illustrious Hongkong squad teammates when he broke the 400 metres medley national junior record at the Kowloon Park pool yesterday. The St Joseph's College pupil, competing in the 13 and 14 years division at the HKASA international invitational age group competition, underlined his potential by winning the medley gold in five minutes 0.46 seconds. It clipped 0.91 of a second off the Hongkong junior record set by Arthur Li Kai-yin, a competitor at the 1992 Barcelona Olympic Games, at the British Age Group Championships five years ago. Kwok was by far the territory's most outstanding swimmer in the three-day gala, having also produced a new meeting record for the 200 metres medley on Saturday. Hongkong coach Chan Yiu-hoi was extremely pleased with Kwok's performance at the meeting during which he walked off with five medals - three golds and two silvers. ''Billy did very well indeed when you're looking only at the results but I think Snowie Pang and Robyn Lamsam also did remarkably well at this meeting,'' said Chan, who will lead the swim team to the East Asian Games in Shanghai next month. ''Unlike Billy, who tapered his training for this competition, Snowie and Robyn have kept up their endurance training as they are preparing for the East Asian Games,'' added the coach. ''I'm very pleased that they both posted very respectable times for their own events and they should do quite well in Shanghai.'' Fifteen-year-old Lamsam yesterday won the 50 metres freestyle in 27.50 seconds while Pang, who was also competing in the 15 to 17 years age category, clocked two minutes 45.95 seconds in her 200 metres breaststroke win.