Tough-as-nails tennis champion Serena Williams is spending her off-court time learning to become — a nail technician. She wrote on her Global Grind blog: “No one likes getting their nails done more than I do . . . I go every four days to get a manicure and every seven days for a pedicure. So, I had a brilliant idea to get certified to be a nail tech.” Williams, winner of 12 grand slam singles titles, has enrolled at the Palm Beach Nail School to become a certified nail tech and will have to fork over $55 every two years to renew her license — which shouldn’t be much sweat for a player who has made more than $30 million in tournament prizes. Serena added, “Don’t be surprised if you hear about a part-time job I’ve picked up at a local nail salon.”