Federer was chasing Nadal for the year-end world number 1, but the Swiss' withdrawal effectively earned Nadal the top position if the latter came through the first match in Paris.
he top-seeded Federer succeeded on the same court where he lost the 2012 and 2013 Basel finals to del Potro.
The 27-year-old Wozniacki's best previous result at the WTA Finals was reaching the final in 2010.
With the trophy, Babos and Hlavackova finished a season, in which they also claimed titles in Rabat, Quebec, Tashkent and Moscow.
Williams, who appeared in the Australian Open and Wimbledon finals this season, won the WTA Finals in 2008 and was also a finalist in 2009.
Roger Federer cruised past David Goffin in straight sets 6-1, 6-2 in the Swiss Indoors semifinals on Saturday to reach an 11th straight final when playing at his hometown event.
The sixth-ranked Wozniacki, a finalist in 2010, leads the tour with most match wins in the season at 59-21 as the Dane heads into Sunday's championship match.
Federer will play either David Goffin of Belgium or Jack Sock of the United States in the semifinals on Saturday.
In the semifinals, Garcia will play Venus Williams and Wozniacki will face Karolina Pliskova.
India's singles challenge ended at the Vietnam Open following the defeat of Yuki Bhambri.
Hingis amassed five Grand Slam singles titles, 13 Grand Slam doubles titles, and seven Grand Slam mixed doubles titles during her stop-start career.
Top-seeded Federer cruised to a 6-1, 6-3 win over 40th-ranked Benoit Paire in 57 minutes.
The 37-year-old Williams advanced from the White Group along with Czech Republic's Karolina Pliskova, who lost to French Open champion Jelena Ostapenko 6-3, 6-1.
The 20-year-old Ostapenko, who won the French Open this year and was making her WTA Finals debut, finished with a 1-2 record in the White Group.
Wozniacki is now 2-0 at the season-ending tournament, while Halep is 1-1.
Prajnesh will lock horns with top American Taylor Fritz, who knocked out India's Vijay Sundar Prashanth.
Federer never faced a break point in the 61-minute match and won every point when his first service landed safely.
Karolina Pliskova comfortably beat Garbine Muguruza 6-2, 6-2 in the round-robin stage to book a place in the semifinals of the showpiece event.
Earlier, Djokovic was expected to make a return in 2018, but the former world number one has decided to start his season early at the Mubadala World Tennis Championship, beginning in Abu Dhabi on December 28.
Groth, an Olympian and Davis Cup mainstay, will call curtains to his 13-year-long career after the Australian Open, which is to be held from January 15 to January 28 next year.
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