She has faced Pliskova twice, losing both matches, and played Williams once - also losing that match.
The Spaniard, though, brushed aside the French Open victor 6-3, 6-4 to join Pliskova at the top of the White Group standings.
Simona Halep says she is relishing being the world number one after delivering an ominous warning to her rivals at the WTA Finals with a crushing, straight-sets victory over the in-form Caroline Garcia on Monday. "She just couldn't find a break or find a way to break me".
Another break of serve soon followed and although Garcia finally held in the seventh game, Halep would not be denied and sealed the contest on her first match point when the Frenchwoman sent a routine volley at the net into the tramlines.
This time previous year, Caroline Wozniacki was on holiday in "quite a few places", her season over after losing in the quarter-finals of the Luxembourg Open and her eventual year-end ranking No. 19.
She forced Muguruza to wait as the Spaniard served for the set, saving three match points, but Mugurza kept calm to secure victory.
In her opening round robin match at the eight-player season finale, Halep halted Garcia's 11-match winning streak with a 6-4, 6-2 masterclass in 88 minutes at the Singapore Indoor Stadium.
"I think I was fighting very well and my shots were there in the important moments". "But in the second set I already started to feel that I was dominating the match and I got more confident".
Earlier Pliskova, who was world number one for seven weeks in the summer, sped past world number eight Williams in the first match of the day.
Williams has appeared at the end of season event on five occasions and won the title in 2008.
The Wimbledon finalist struggled with her serve against Pliskova, hitting six double faults and making 19 unforced errors to Pliskova's 12.
Pliskova started the match in trademark fashion with an ace and showed her proficiency in returning by breaking Williams in the second game to gain an early stranglehold.
"I don't know how she felt but I think I was trying to take that advantage that I was here a year ago".
"Maybe the score looks a little easier than it was". "I'm very disappointed with the performance today".