IT had been a tough match on a scorching day when a perspiring Paul Selway-Swift beat Cynthia Rich in the final of a biannual tennis tournament for a total prize of $1.3 million - all going to charity. The executive director of the Hongkong Bank, who although fit and lean was not, he admitted, the top pick for a winner, had one secret weapon: his doubles partner was Colin Grant, Hong Kong Dunhill Cup winner. ''The final was tough,'' he said, of the match in which his team battled against the athletic wife of Jardines MD Nigel Rich and tennis coach Hans Franklin for the informal Save the Children Fund cup. ''But what was really tough was telling my colleagues who had sponsored me that they had to pay up the maximum amount of sponsorship money.'' Some colleagues, believing that Mr Selway-Swift would get no further than the first round, ended up being a little more generous than they had intended, and paid up several thousand dollars a head, he said. Even that didn't make Mr Selway-Swift the winner in the sponsorship stakes: his total of $64,000 was overshadowed by the impressive efforts of General Chamber of Commerce chairman Paul Cheng, who helped to add $320,000 to the fund. ''I don't know about myself, but I think I'll have to get my sponsors in training for next year's event,'' Mr S-S said.