The House of Lords ruled sports stars who get paid for endorsement deals and appearances in the UK are liable for income tax.
Agassi had disputed a $50,000 tax assessment for the 1998-99 year. It centred on cash he had received from sportswear firms Nike and Head and paid to his US-based Agassi Enterprises company.
The decision is a relief to the UK’s Inland Revenue – had it lost the case it would be open to refund millions to entertainers and sports stars who had paid tax on their deals.