British number three Kyle Edmund let slip a 5-1 final-set lead as American Jack Sock surged back to win their Paris Masters second-round clash 4-6 7-6 (7-4) 7-6 (7-5).
Edmund, who won his only previous meeting with Sock in July, twice served for the match.
But Sock, ranked 22nd in the world, trained his power on Edmund's weaker backhand wing to fight his way back.
Edmund saved two match points in the final tie-break before losing a third.
It is the final match of the 22-year-old Yorkshireman's 2017 campaign and comes after an impressive run to the semi-finals in Austria last month that included a victory over David Ferrer.
He is expected to finish the year ranked 49th in the world.