Paris - Dominic Thiem faces an anxious wait to see if he qualifies for the Tour finals after slumping to a 6-2, 6-4 defeat against Jack Sock at the Paris Masters on Wednesday.
Thiem entered this week's tournament occupying one of the final two qualifying spots for London, but his second-round exit has left his place in the eight-man field vulnerable.
The Austrian dropped his opening service game against Sock, setting the tone for a dismal afternoon as the American blew away the sixth seed in under an hour.
Thiem has won four titles this season and reached the semi-finals of the French Open, but his subsequent dip in form could prove costly with six players still fighting for the remaining two London berths.
Marin Cilic, a winner in Basel at the weekend, further bolstered his chances of playing at the November 13-20 Tour finals by defeating compatriot Ivo Karlovic 7-6 (9/7), 6-2.
With Rafael Nadal calling an early end to his season, the Croat is eighth in the Race to London standings and will face David Goffin for a place in the quarter-finals.
Belgian Goffin is still in the hunt after beating Nicolas Mahut 7-6 (9/7), 6-2, while 2008 Paris champion Jo-Wilfried Tsonga cruised past Albert Ramos-Vinolas 6-3, 6-4.
French 13th seed Lucas Pouille is out of the running, though, despite rallying to beat Feliciano Lopez 6-7 (1/7), 6-3, 6-4.
Pouille had arrived in the French capital with very slim qualification hopes but Cilic's earlier victory eliminated him from contention.