user: ### NOW ANSWER:
Question: Who won the Men's Doubles when Guo Xin won the Women's Singles?
Columns: ['Year', 'Mens singles', 'Womens singles', 'Mens doubles', 'Womens doubles', 'Mixed doubles']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1999.0, 'Marvin Steve', 'Maja Pohar', 'Dmitry Miznikov Valery Strelcov', 'Natalja Esipenko Natalia Golovkina', 'Valery Strelcov Natalia Golovkina']
SQL:

assistant: SELECT "Mens doubles" FROM "Table" WHERE "Womens singles" = 'Guo Xin';