German Alexander Zverev's 10-match winning streak was ended by unseeded American Francis Tiafoe in the second round at Cincinnati Open.
Zverev, 20, has moved up to seventh in the world rankings following victory over Roger Federer in Montreal.
But he was beaten 6-4 3-6 4-6 by the 19-year-old world number 87 Tiafoe.
In the women's event world number nine Venus Williams was also knocked out in the second round by Australian qualifier Ashleigh Barty.
Barty, ranked 48 in the world, beat the Wimbledon finalist 6-3 2-6 6-2 to earn a first career victory over a top 10-ranked player.