Wimbledon champion Muguruza was equally happy to get her title bid off to a winning start with victory over French Open victor Ostapenko, the youngest player in the eight-woman field.
But the match was an anti-climax for Williams and an early blow to the 37-year-old's bid to be crowned the world's number one for the first time in 15 years.
Elina Svitolina won five tour title this season to qualify for the first time, although the Ukrainian struggled to replicate her top form at the grand slams, while Denmark's Caroline Wozniacki is making her fifth WTA Finals appearance.
As the top eight WTA players of 2017 gather for the season finale in Singapore, it is the most famous female tennis player of all - Serena Williams - who has been central to all their aspirations. I think it was very tough - from the baseline we were both playing pretty fast and pretty good shots so it was an fantastic match I think.
"Well, I think it's very important to start with a win", the Wimbledon champion told reporters.
While it was clear that Williams was the one whom the audience was more excited to watch, it was her opponent Karolina Pliskova who had more to cheer about at the end of their 73-minute encounter.
"I wanted to win so badly and she was just not giving me anything", Muguruza added after sealing victory on her third match point.
"I thought it was normal that she's playing incredible and I'd kind of wait for my moment, until I serve and try to take control.it went well". "I look forward to continue working hard to keep this position".
I beat her once from match point and once I lost a very close match. "(But) I'm going to fight for that".
"Just the depth in the game". Ostapenko will play Williams in the next White Group match tomorrow while Muguruza will face Pliskova.
"The competition is so much greater", said the seven-time Grand Slam victor in singles.
"So this tournament is really going to decide who is going to finish the year world number one".
Williams, whose stony expression remained in the post-match press conference, rued her missed chances yesterday.