The 37-year-old Williams said "it's hard to produce your best tennis at the end of the year because you've given so much."
Williams holds a 7-0 winning career record over 27-year-old Wozniacki and has only lost one of 15 sets they've played.
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. Wozniacki was a year-end finalist in 2010.
Williams started the semifinal flat but kept improving against Garcia.
"One more match and I'm going to be playing against one of the best players in the tournament, another Caroline, and it's going to be like deja vu," Williams told the crowd.
"This tournament is one of the best, the top eight players in the world. Nothing is easy and to be standing in the winner's circle, almost, is so amazing."
Williams became more aggressive and Garcia, making her WTA Finals debut, started to make too many errors. In the end, Williams saved 12 of 13 break points in the match, and took advantage of four of nine break-point opportunities.
"I think in the second and third (sets) I was able to figure it out a little more," Williams said.
Garcia called it a "very difficult" match.
"She's hitting the ball very hard, very aggressive, huge serve, sometimes no rallies," Garcia said.
Sixth-ranked Wozniacki leads the tour with most match wins on the season at 59-21 so far.
The third-ranked Pliskova's loss is good news for Simona Halep, who will retain the No. 1 ranking through to next year.
Pliskova, playing in the year-end semifinals for the first time, needed to win the WTA Finals title to return to the No. 1 ranking for a second time this year.
Wozniacki improved her career record over Pliskova to 6-3, and is 3-2 in meetings this year.
Both players had six set points in the first set, but it was Wozniacki who capitalized on the sixth offering when Pliskova netted a backhand.
All six of Wozniacki's set points were in the tiebreaker where she initially led 6-1. Pliskova held her first three set points on Wozniacki's serve in the 10th game and three in the tiebreaker.
"All of a sudden it's 6-2, 6-3, 6-4, and I said this isn't fun anymore," Wozniacki said. "I started thinking I should have lost this set already, so actually this is just a bonus. So I had a lot of talks with myself at that point."
From 3-3 in the second set, Wozniacki won 12 of the last 15 points to take her place in the final.
Hingis and Chan won nine titles this season, including the U.S. Open trophy.
"I think it's been an amazing journey and amazing career that I can be proud of," the 37-year-old Hingis said. "It's not really goodbye. I hope I'll still be part of the game."