David Goffin recovered from a slow start to beat Austrian Dominic Thiem 6-4 6-1 and set up a last-four clash with Roger Federer at the ATP Finals.
Belgian Goffin, thrashed by Grigor Dimitrov two days ago, appeared to be feeling the effects of that mauling as he went 3-0 down with a whimper, but came alive to win 15 points in a row during a first-set comeback.
Thiem looked disinterested at the start of the second set and quickly went 4-1 down. There was some brief resistance in a 10-minute sixth game but Goffin held and claimed victory on his second match point with a searing backhand winner.
A scrappy contest saw Thiem rack up 27 unforced errors to Goffin's 20, with the combined winner count only 21.
Goffin's victory means three of the four players who had qualified for the elite tournament for the first time have progressed from the round-robin stage. The other semi-final will pit Dimitrov against American Jack Sock.
Dimitrov is up against alternate Pablo Carreno Busta of Spain in Friday's late match which is academic apart from the prize money and ranking points available.