WASHINGTON — Brian McNamee described for jurors a relationship with Roger Clemens that had the hallmarks of an illicit affair — except their secret was steroids.

“Roger would ask me, `What are you doing? Are you available tonight?’ I knew exactly what he was talking about,” McNamee said Monday, in the first day of his testimony against his former client and friend.

Back then, in 1998, Clemens was pitching for the Toronto Blue Jays, and McNamee was the team’s strength and conditioning coach. According to McNamee, the two men went to Clemens’ apartment in the Jays’ Skydome stadium.

“Roger pulled down his pants, exposing his right buttocks cheek to me,” McNamee said. A few seconds later, Clemens said he was ready. McNamee said he then “plunged the fluid in, into his buttocks.”

“That,” McNamee said, “was the first time I injected Roger Clemens.”