NOBODY knows better than Walter Swinburn that a top-class filly, in top form, can take an awful lot of beating in big races such as today's Hongkong International Cup; so it is easy to understand his enthusiasm for the French-trained Urban Sea. Over the years, Swinburn's name has been linked with a series of fillies right out of the top draw: Sonic Lady, Milligram, and the Classic winners Musical Bliss, Unite and Hatoof. All were outstanding on their day. Under the present, special conditions of the big Hongkong race, Urban Sea need not necessarily face a challenge of Classic proportions, but she will still have to run right up to her very best form, seen last year in France and Canada, if she is to win. Swinburn, forced to by-pass yesterday's Greenham Stakes meeting at Newbury to get to Hongkong in time for today's fixture, has no doubt he has made the right move. ''I went over to France to ride Urban Sea in a gallop recently, and she gave me a tremendous feel, she moves very well,'' Swinburn said. ''She has a lot going for her in this race. She is very tough, and she travels so well during a race, which I was able to witness first-hand in Canada last season.'' Urban Sea rated as one of the biggest improvers towards the back-end of 1992, and it was remarkable that she never actually landed a Group One or Group Two prize. Her huge prize money haul in the Piaget d'Or at Deauville was a ''big'' win without a penalty, as far as today's race is concerned. After finishing sixth in the French Oaks at Chantilly in June, she made rapid progress in the autumn to finish third, beaten just over half a length, behind the classy Jolypha in the Prix Vermeille at Longchamp, and then second, only one and a quarter lengths behind Hatoof in the E. P. Taylor Stakes at Woodbine. ''That is real Group One form,'' Swinburn very accurately points out. ''Hatoof won the 1,000 Guineas at Newmarket in the spring, and was a very high-class filly indeed. I rode her again when she won at Woodbine, and I thought Urban Sea ran a fantastic race to get so close to her.'' Back in the 1960s, races such as the Washington International, as it was then known, were something of an oddity; an international challenge to be undertaken only by the very adventurous. Nowadays, there are at least a dozen races worldwide, attracting overseas horses. Hongkong is the newcomer in this field, but being so centrally located geographically, it has the opportunity to give Japan a run for its money. But one factor that will never change is the necessity for horses to cope with the travel. The flight-time from Europe may not be as long as from the United States, but horses need to handle it, just the same. ''Urban Sea has proven she can travel well. The journey to Canada was demanding and yet she arrived there in good order, ready to run a big race,' Swinburn said. ''I think she must have a very big chance, although River Verdon must be the one to beat,'' he added. There is little doubt that Urban Sea dominates the European challenge for the Cup. She is the highest-rated of the trio, and with word reaching me that she has travelled well, she has an undeniable chance. Mellottie is one of the true fairytale stories of the British turf, rising from relative obscurity to be challenging for this very valuable prize in Asia. The two factors that weigh heavily against him are that his best form has been on straight courses,and that he has had back problems since arriving in Hongkong. Half A Tick is a true Group Three performer, who seems to run his best races smothered up in the field with the pace on up front. Richard Quinn, his much-travelled and successful partner, will be attempting to get him in the middle of the pack as the field settles down before, hopefully, coming with a late run.