A wildly unpredictable match veered one way, then the other, before Romanian Halep ended Kerber’s resistance on her fourth match point in a spellbinding deciding set that pushed both players to the limit of their endurance. Former champion Kerber, who won only five points in losing the opening five games, was jelly-legged at times but fought off two match points in the 10th game of the third set before seeing two of her own go begging two games later.