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.
Rafael Nadal and Roger Federer may have returned to major winning ways this year, but the Western & Southern Open has offered a tantalising glimpse into the future.
Kei Nishikori is the latest top 10 player to withdraw from this month's US Open because of injury and has confirmed he will miss the rest of the season.