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Venezuelan-born Muguruza won the French Open in 2016 and Wimbledon in 2017, and she reached the final of the Australian Open in 2020.
Muguruza defeated Anett Kontaveit 6-3, 7-5 to become the first Spaniard to win the WTA Finals title.
In their first career meeting, Muguruza's superior firepower put Badosa on the back foot from the outset.
Karolina Pliskova missed the cut despite her comeback victory over Barbora Krejcikova.
The Spaniard, a former world No.1, was down a set and a break but fought back strongly to beat Jabeur 3-6, 6-3, 6-0 to claim her second title of the year and ninth overall.
Osaka battled back from the brink to reach the quarterfinals.
Defending champion Nadal was stunned by Argentine Diego Schwartzman in the quarterfinals.
The 21-year-old Kenin, seeded 14th at Melbourne Park, converted her second match point against the twice Grand Slam champion, who double faulted on the final point, to end an engrossing contest.
The 26-year-old Muguruza would not have been on many lists of pre-tournament contenders but the class and fighting spirit that propelled her to the 2016 French Open and 2017 Wimbledon titles has come flooding back in Australia.
Muguruza stormed back into the Grand Slam big time with an emphatic 7-5, 6-3 win over Russian Anastasia Pavlyuchenkova to reach her first Australian Open semifinal.