Federer already clinches ATP finals spot; Jack Sock snaps long U.S. skid

London • A day after the ATP Finals lost Rafael Nadal to injury, Roger Federer lifted spirits at the O2 Arena by extending his stay to the weekend.The 19-time Grand Slam champion guaranteed his place in Saturday's semifinals by defeating Alexander Zverev 7-6 (6), 5-7, 6-1 on Tuesday.The top-ranked Nadal's withdrawal on Monday meant this was a clash between the two highest-ranked players left in the elite season-ending event, and it was predictably close.Federer looked in control when he bro