The American 16th seed, who started the week in 24th place in the race to the season-ending tournament in London, needs to win the title against Filip Krajinovic to take part in the Nov. 12-19 event after the Serbian qualifier beat another American, John Isner 6-4 6-7(2) 7-6(5).
Sock is looking to snatch the last remaining spot for the eight-man tournament at the O2, which is currently held by Spain's Pablo Carreno Busta.
"I feel more seized than the pressure. If the pressure would have kicked in, a lot of it would have been today as well playing in front of the French crowd, playing a French player," Sock told a news conference.
"But I was able to go out today and play my game, swing big, kind of like I always do, and play some good tennis and get through.
"And tomorrow is just a good opportunity for me to go out and try to compartmentalize all the things that would come with winning tomorrow and just go out and play my tennis again."
Sock broke decisively for 6-5 in the opening set when he benefited from a net cord.
Benneteau, who benefited from a wildcard entry, was broken twice in the second set, each time serving a double fault on break point as Sock cantered to victory.
Earlier, Krajinovic ended ninth seed Isner's hopes of reaching the ATP Finals.
The world number 77 was 3-0 down in the deciding-set tiebreak but he kept his composure to prevail on the first match point.
Isner would have booked his place in the London field if he had won the title here.
The American had his right leg massaged twice by the trainer but it did not prevent him from firing down 31 aces, although it was a perfect service return that gave Krajinovic his first match point, which he converted after 2-1/2 hours.
Krajinovic, who benefited from world number one Rafa Nadal's withdrawal in the quarter-finals, held serve throughout and relied on his quick backhand to unsettle Isner.
Krajinovic made only four unforced errors in the opening set, saving three break points, and broke on his only opportunity to take a 3-2 lead with a fine service return and closed it out when Isner buried a forehand in the net.
Isner breezed through the second-set tiebreak and he seemed on his way to winning another tiebreaker in the third set when he opened up a 3-0 lead.
Krajinovic had other ideas as he won seven of the nine following points to reach his first ATP final, triggering wild celebrations in his camp.
(Reporting by Julien Pretot; Editing by Gene Cherry)
