Amanda Anisimova slipped to a straight-sets defeat to Anna Kalinskaya as her disappointing run of form continued.
A month on from her 6-0 6-0 loss to Iga Swiatek in the Wimbledon final, Anisimova failed to get going in her first match at the Cincinnati Open.
Having received a walkover in the round of 64, fifth seed Anisimova failed to find her level as she lost 7-5 6-4.
Anisimova won two matches at the Canadian Open, including beating Emma Raducanu, earlier this month, but then lost to Elina Svitolina in the round of 16.
There was no such trouble for Anisimova's fellow American Madison Keys, as the reigning Australian Open champion swatted aside Japan's Aoi Ito.
Keys needed just over an hour to triumph 6-4 6-0.
Next up for Keys is an enthralling clash against ninth seed Elena Rybakina, who had a tougher time against Elise Mertens but nevertheless prevailed 4-6 6-3 7-5.
Data Debrief: Resurgent Rybakina
Rybakina has now had to come from behind in both of her matches at the Cincinnati Open.
It is the first time she has claimed multiple comeback wins at a single WTA Tour-level event since 2023, when she did so at the Canadian Open.
Since the format's introduction in 2009, meanwhile, Keys (45) only trails three American players for WTA-1000 match wins on home soil – Serena Williams (61), Venus Williams (52) and Sloane Stephens (48).
