Muguruza (2-0) came into the tournament on the back of her first premier level tournament win in Beijing at the start of the month and backed up her impressive opening round-robin win over Safarova with another straight sets victory over Kerber.
Kvitova (1-1), Kerber (1-1) and Safarova (0-2) can all still advance to the last four from the 'White Group' with one round of matches to play but Muguruza is in prime position to advance as the winner of the four-player pool in Singapore.
"Every time I step on the court against Angelique it seems to be a tough match so I knew I had to be at my best today to secure the victory," Muguruza said in a courtside interview after improving to 4-3 against the German.
The 22-year-old Muguruza started the contest full of positive intent, immediately finding the corners and licking the paint with her powerful ground strokes as she broke in the first game before Kerber had a chance to settle.
Slowly but surely, the German worked her way back into the set, breaking back in the fourth game through sheer determination and a tenacious defence as the pair fought tooth and nail to gain the ascendancy.
At 4-4, the Spaniard increased her intensity levels a notch and quickly forged three break points, edging ahead on the second of them and closing out the set with a blistering serve in the next game that Kerber sent sailing over the baseline.
Muguruza, who started the season ranked outside the top 20, continued to apply pressure at the start of the second as Kerber hung on by her fingernails, until the Spaniard finally converted her sixth break point of the set to edge 3-2 ahead.
Kerber refused to buckle, however, and broke back against the run of play in the eighth game before Muguruza wrestled back control in what had become a baseline slugfest, then closed out the contest as her demoralised opponent wilted under a barrage of booming forehands.
CZECH MATES
Earlier, Kvitova got her campaign back on track with a hard-fought victory over Fed Cup team mate Safarova to record her first win of the week on the Singapore Indoor Stadium's purple hard court.
Both women had lost their openers on Monday so were desperate to pick up a much-needed win and it was the double Wimbledon champion who made the most of her opportunities on the key points to emerge victorious.
"We actually are good friends from the Fed Cup, so I'm really glad the (final) is coming soon and we are going to be colleagues and not opponents," Kvitova said of the upcoming Fed Cup showpiece against Russia in Prague on Nov. 14-15.
"It's tough to play her... not only because she's like a very good player right now, in good form, but also playing a friend is a little bit tougher with emotions and everything.
"So it's not easy to handle all this kind of stuff."
The left-handers took to the court in identical outfits and were also almost impossible to separate on the scoreboard until Kvitova ramped up the pressure to break in the 12th game of both sets to improve to 8-0 against Safarova.
The 'Red Group' concludes on Thursday with Simona Halep (1-1) taking on Agnieszka Radwanska (0-2) before Maria Sharapova (2-0) faces Flavia Pennetta (1-1) in the night match.
(Editing by Patrick Johnston and Ken Ferris)
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