Williams came out on top of a tight contest that went all the way to a third-set tie-break despite her winning five games on the bounce, letting the Belgian back in by missing six match points on her serve in the eighth game.

She missed a further two match points as Mertens battled to level the set at 6-6, before finally taking the match in the tie-break and setting up a headline-grabbing clash with her 10th-seed sibling.