There was a big upset at the Delray Beach Open after second seed Juan Martin del Potro suffered a last-16 loss to wildcard Frances Tiafoe.
Back in the top 10, there were high hopes for Del Potro this week but the 2011 champion was upstaged by 20-year-old American Tiafoe 7-6 (8-6) 4-6 7-5 Thursday.
Tiafoe showed glimpses of his raw talent against Del Potro in his straight-sets loss to the Argentine star at the Australian Open last month.
But Tiafoe produced a performance beyond his years to reach the quarter-finals, firing down 17 aces and saving 14 of 16 break points as he thwarted the 2009 US Open champion.
Standing in the way of Tiafoe and the semi-finals is South Korean star and eighth seed Hyeon Chung.
Chung, who sensationally reached the Australian Open semis, was too good for qualifier Franko Skugor 6-4 7-6 (7-4).
Hard-hitting Canadian Denis Shapovalov also advanced with a 6-7 (6-8) 6-4 6-4 win over Jared Donaldson.
Shapovalov – the youngest player in the draw at 18 years of age – needed two hours, 25 minutes to see off the American.
Next up for Shapovalov is Taylor Fritz, who defeated veteran Mikhail Youzhny 6-1 4-6 6-3.