NEW YORK – The chant started late Friday afternoon among Juan Martin del Potro’s Argentine friends – a soccer chant, of course, rising from his hometown cadre: "O-le! O-le! O-le! O-le! Del-po, Del-PO!" When they repeated it, there was every reason to think it would stay contained, begin small and end there, with their hero up 6-3 in the first set tiebreak of his U.S. Open semifinal match against Rafael Nadal. Why not? Until then, a good half of Arthur Ashe Stadium had cheered the defending champion’s every move.