American Jack Sock will start a rank outsider for the season-ending ATP Finals after clinching the last spot in the eight-man field headed by superstar veterans Rafael Nadal and Roger Federer.
World No.22 Sock qualified with a 5-7 6-4 6-1 win over Serbian Filip Krajinovic in the Paris Masters final on Sunday.
He is the sole player outside the top-10 to reach the London tournament from November 12-19, which is looming as a showdown between world No.1 Nadal and No.2 Federer, who split this year's grand slam titles two apiece.
Wimbledon finalist Marin Cilic and world No.4 debutant Alexander Zverev will also provide stiff competition at The O2.
However the slog of the long tennis season has taken its toll with some notable absentees.
World No.7 Novak Djokovic, a four-time winner of the tour finale, remains sidelined after ending his season in July to treat an elbow injury.
Third-ranked Andy Murray and Swiss star Stan Wawrinka are the other top-10 players to sit out, with their seasons cruelled by hip and knee injuries respectively.
Sock, who started the week in 24th position in the Race to London, needed to win the Paris title to leapfrog Spain's Pablo Carreno Busta.
He wore down the defence of world No.77 Krajinovic, becoming the first American to win the title since Andre Agassi in 1999.
"There have been a lot of firsts. It started at the French Open with my first fourth round of a Slam, now I've won my first Masters 1000 in Paris," Sock said.
"This will be my first time in the top 10 and this will be the first time making the year-end Finals."
He is the first American man to make it into the singles draw of the elite season-finale since Mardy Fish in 2011.
"I obviously snuck into ... London unexpectedly so zero pressure on me to go there," Sock said.
"I'm going to swing big and play my game, like I always do, and just honestly enjoy the moment."
2017 ATP FINALS (BY WORLD RANKING)
1. Rafael Nadal (Spain)
2. Roger Federer (Switzerland)
4. Alexander Zverev (Germany)
5. Marin Cilic (Croatia)
6. Dominic Thiem (Austria)
8. Grigor Dimitrov (Bulgaria)
10. David Goffin (Belgium)
22. Jack Sock (US)
