Garcia covered her face with her hands and fell on her back after a Sabalenka forehand sailed wide on match point to earn her fourth title of the year.
"Today was such an intense final," a smiling Garcia told Tennis Channel while donning a cowboy hat.
"I'm very proud of how I was able to keep my mind calm and really focus on one point at a time, one game at a time."
Both players' thunderous serves were firing early and neither faced a break point in the first set of the final of the season-ending tournament, which features the WTA Tour's top eight players.
But in the tiebreaker, the Belarusian's second serve, which has bedevilled her at times this season, failed her again - first a costly double fault to fall behind 5-2 and then another on set point.
Garcia kept the pressure on from there, breaking to open the second set and coming through a tense deuce game to seal the win in a match where she fired down 11 aces and never faced a break point.
After receiving the Billie Jean King trophy from American great Chris Evert, Garcia had kind words for Sabalenka, the tournament's seventh seed who stunned world number one Iga Swiatek in Sunday's semi-final.
"You bring so much energy on the court every time, it's always a pleasure to watch," Garcia said on stage at the indoor hardcourt tournament.
"It's great to have all your energy in women's tennis. We really appreciate it."
Garcia was ranked outside the top 70 in mid-June but has since won four titles, made the semi-finals of the U.S. Open and will now rise to a career-high number four.
"It's super important to keep improving," she said.
"If you don't move forward, you're moving backwards. And that's not something we want to do on this team."
Sabalenka, who will rise to world number five, looked forward to more title deciders against Garcia.
"Caro, I really hate you right now but you played unbelievable tennis, not only tonight but this year," Sabalenka said.
"Congrats on the results and hopefully we'll play many more finals."
