IT'S all systems go for Hong Kong's historic racing mission to Japan next month following yesterday's top effort from Winning Partners in the Sassoon Challenge Cup at Sha Tin. The International Bowl hero humped 138 pounds into second behind Carry On Winning in yesterday's feature 1,200-metre event after which he was confirmed a definite starter in the HK$14 million Yasuda Kinen, run over a mile at Tokyo racecourse on May 15. He will become only the second Hong Kong-trained horse to race outside the territory following River Verdon's disappointing trip to Chicago for the Arlington Million at the start of last season. Trainer Neville Begg said: ''He had to run well today to justify going to Japan and this run was certainly more than good enough. ''The owners have confirmed they want him to race in Japan.'' The experienced Begg added: ''Ordinarily I wouldn't have run him under such a big weight today, conceding so much to most of the field. ''But he had to run as it was the last opportunity to get a race into him before Japan. ''It was a big effort considering he's never really carried such a weight before and he can now go there with race fitness on his side.'' Winning Partners will leave for Japan next Saturday in order to meet quarantine arrangements and Begg will follow him out after the Chairman's Prize meeting on May 5. He will be ridden in the Yasuda Kinen by former Japan Cup-winning jockey Lance O'Sullivan who was in the saddle yesterday. O'Sullivan was chuffed to bits by the display, having been worried about Winning Partners' condition going into the race. ''Now I know what a top racehorse he is,'' stressed O'Sullivan after Winning Partners had finished 11/4 lengths second to Carry On Winning to whom he was giving 22 pounds. ''And he's gone out and put up this cracking effort which leaves me really looking forward to Japan.'' Begg reckons the move up to a mile should suit his International Bowl winner, pointing out: ''He settles well and gives me the impression that the distance will not be a problem at all.'' Winning Partners will have to take on some of the world's best milers in Japan including English Classic winner, Sayyedati, who took last season's 1,000 Guineas for Clive Brittain. Meanwhile, Carry On Winning's victory capped a big day for local handler Wong Tang-ping who was also on the mark with outsider Miss Piggy in the Indian Recreation Club Challenge Cup to complete a prestigious and lucrative Cups double.