David Goffin's hopes of securing a place in the ATP World Tour Finals were dealt a blow as he suffered a surprise last-four defeat to Diego Schwartzman at the European Open.
Goffin sits 10th in the Race to London and would have been expected to get the better of world number 77 Schwartzman on Saturday.
However, the top seed in Antwerp was unable to convert two match points when serving with a 5-4 lead in the third set.
Schwartzman, winless in his previous 11 matches against top-20 players, then won 10 of the last 11 points to complete an unexpected 7-5 2-6 7-5 triumph.
The Argentinian, who claimed his maiden title in Istanbul earlier this year, will now face Richard Gasquet in the final.
Gasquet came from a set and a break down to beat rising star Kyle Edmund 3-6 7-5 6-2.
Sunday's Kremlin Cup final in Moscow will be contested by Fabio Fognini and Pablo Carreno Busta.
Fognini beat Philipp Kohlschreiber 6-1 7-6 (7-2) to move within one win of a maiden hard-court title on the ATP Tour, while Carreno Busta also won a second-set tie-break against Stephane Robert to reach his fourth final in 2016.