FRANKFURT, Dec 20 (Reuters) - German sports goods maker Adidas (ADSG.DE) is confident about its business next year and expects its Reebok and Adidas brands to grow, its chief executive told a German newspaper.

"I am cautiously optimistic for 2010," Herbert Hainer was quoted as saying by Handelsblatt in an e-mailed excerpt of an article ahead of publication on Monday 21 December. "The Reebok and Adidas brands will grow next year," he said, also confirming the company's 2009 outlook for a sales decline at a single-digit rate.

Adidas, the world's second largest sports goods maker after Nike (NKE.N), said in November it expected the soccer World Cup in South Africa next year to boost consumer demand for sports gear, anticipating record sales of soccer-related products of more than 1.3 billion euros ($1.87 billion).

Hainer also told Handelsblatt that the company continued to reduce its debt during the fourth quarter and that it further decreased its inventories.

(Reporting by Christoph Steitz; editing by John Stonestreet)