The American took the last spot in the ATP Tour finals after defeating Serbian qualifier Filip Krajinovic 5-7 6-4 6-1 to win the Paris Masters title on Sunday.
The sixteenth seeded Sock, 25, is the first American to win the competition since Andre Agassi in 1999.
Jack, who started this week in 24th position in the Race to London, is expected to win the title to leapfrog Spaniard Pablo Carreno Busta into eighth place.
He did only that with a great performance, bringing down the defense of world number 77 Krajinovic.
Sock won his initial two-game service game to love yet kept running into a spot of trouble in the 6th game. He served and volleyed to spare a breakpoint however a twofold blame gave Krajinovic a break and a 4-2 lead.
The American crushed straight spirit yet a fine forehand earned the Serb a set point and Sock made a wreck of another forehand as his rival, who shielded well, took the lead.
Krajinovic, nonetheless, lost his concentration and a forehand mistake offered Sock an early breakthrough in the second set. The Serb fell 4-1 down and could just crush his spirit once, Sock leveling for one-set all.
The hard moment passed and Sock, who will complete the year as the American number one out of the blue, dashed through the choosing set to win the title.