By Jim Forsyth

SAN ANTONIO (Reuters) - Retired NBA superstar Tim Duncan's former financial adviser has been indicted on federal fraud charges accusing him of bilking the long-time San Antonio Spurs player out of millions of dollars, court papers unsealed on Friday showed.

Charles Banks, 49, was indicted by a grand jury on two counts of federal wire fraud on suspicion of defrauding Duncan, U.S. prosecutors in Texas said. Banks headed a Colorado-based business called Gameday Entertainment.

The indictment identified Duncan, 40, only by his initials, but his lawyer confirmed that it referred to the five-time NBA champion and that the amount misappropriated by Banks through a series of financial schemes totaled $7.5 million to $13 million.

"There are two counts of wire fraud where Mr. Banks sent documents to Tim, and Tim returned them with his signature," Tullos Wells, long-time outside counsel for the Spurs and an attorney for Duncan, said in an interview. "Mr. Banks had misrepresented what those documents were."

Wells said Banks tricked Duncan into committing money to guarantee loans from financial institutions for several of his businesses, many of which Wells said Banks knew were failing. Those businesses included a beauty supply company, a winery and hotels, according to court documents.

Banks made an initial appearance in court in San Antonio on Friday and was released on bond. If convicted, Banks faces up to 20 years in prison on each count.

"He is innocent of these charges and confident that when all the facts and circumstances are brought to light, he will be exonerated of any wrongdoing," Banks' lawyer, Johnny Sutton, said in a statement.

The federal indictment said Banks encouraged Duncan to lend $7.5 million to Gameday in 2012. Banks was also accused of asking Duncan to guarantee $6 million in debt for Gameday.

"He had him sign documents that misrepresented what he was getting, in order to acquire some additional bank loans that Mr. Banks needed for his other business interests," Wells said.

Duncan did not realize the alleged fraud until he had to get his finances arranged in 2013 due to a divorce, court papers showed.

Banks is also being sued by Duncan, one of the greatest power forwards in NBA history who retired in June after 19 seasons with the Spurs. Duncan's suit, filed last year in state court, accused Banks of investment fraud.

"Banks exploited my good intention for his personal gain and my substantial loss," Duncan said in a statement earlier this year.

A source close to the civil case said Duncan's total investment with Banks was in excess of $20 million.

Banks has claimed in civil court filings that Duncan received "voluminous documentation" on all of the transactions he was making on his behalf.

(Reporting by Jim Forsyth; Additional reporting by Jon Herskovitz in Austin, Texas; Editing by Cynthia Osterman and Will Dunham)