Team USA’s Jack Sock and Ryan Harrison fought off a determined effort from a resilient Belgian doubles team to put the U.S. through to the Davis Cup World Group Semifinals for the first time since 2012, defeating Sander Gille and Joran Vliegen, 5-7, 7-6(1), 7-6(3), 6-4, on Saturday to clinch a 3-0 victory for the …