The NFL season starts today, but star offensive tackle Bryant McKinnie was focusing on tennis over the weekend. McKinnie — who’s been in a heated public contract squabble with the Baltimore Ravens — was at Arthur Ashe Stadium on Saturday cheering his close friend and tennis mentor Serena Williams as she advanced to the US Open quarterfinals. According to a spy, McKinnie, an 11-year NFL vet, “loves to pick up a racket on his off days . . . often with Serena and Venus Williams. They aren’t bad tennis instructors to have!” But McKinnie, who also owns record label B Major Music Group, won’t have too much free time to work on his ground strokes. After tweeting yesterday, “Decision is made! I’m gone!” he wound up reaching an agreement with Baltimore and posted he would remain a Raven.