Grigor Dimitrov reached the Western & Southern Open quarter-finals with a comfortable straight sets victory over Juan Martin del Potro in Cincinnati.
Victoria Azarenka may miss the US Open later this month because of a disagreement with her former partner over the care of their baby son.
Venus Williams and Angelique Kerber both suffered early exits in the women's event at the Western & Southern Open in Cincinnati.
Rafael Nadal is through to the third round in Cincinnati after extending his spotless record against Richard Gasquet.
American teenager Frances Tiafoe stunned in-form world No 7 Alexander Zverev with a thrilling three-set victory at the Western & Southern Open.