Argentina's Juan Monaco took advantage of an ailing Jack Sock to rally for a 3-6 6-3 7-5 victory for his second US Men's Clay Court Championship.
The 32-year-old Monaco, also the 2012 winner at River Oaks, held off a surging Sock in the third set after taking a 4-1 lead.
Sock, who battled an aching shoulder and leg cramps, won the next four games, but was broken in the 11th game when he missed an underhanded serve as the humid conditions took their toll.
Early in the match, Monaco also received attention from the trainer for an apparent back issue.
Defending champion Sock said after the match that he'd begun feeling poorly with flu-like symptoms on Saturday night.
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It is Monaco's ninth tour title and first since 2013, eight of which have come on clay.
