Pironkova claimed the Apia International Sydney, going through qualifying and beating three Top 10 players in en route to the title. She defeated ninth-ranked Angelique Kerber in the final Friday.
Muguruza's route to her title was somewhat easier. She did have to qualify for the main draw but had two matches cut short by retirement and didn't face a seeded player until the final, where she beat seventh-seeded Klara Zakopalova.
Muguruza moves up 20 places in the rankings to a career-high No. 38. Pironkova was ranked 31st in September 2010 but saw her ranking slip as she experience health issues. The win Saturday in Hobart boosts her from 107th to No. 57.
Kvitova and Errani were also beaten Monday in the first round of the Australian Open but that won't cost them many rankings points because Kvitova lost in the second round and Errani in the first last year in Melbourne.
Serena Williams sits atop the WTA rankings for a 48th consecutive week and 171 weeks overall. Next week she'll tie Caroline Wozniacki and herself for the 14th-longest number of consecutive weeks at No. 1. Her personal record in that category is 57 weeks.
The WTA Top 10, listing player's name, home country and rankings points:
1. Serena Williams, United States, 13,260
2. Victoria Azarenka, Belarus, 8,151
3. Maria Sharapova, Russia, 6,076
4. Li Na, China, 5,970
5. Agnieszka Radwanska, Poland, 5,470
6. Petra Kvitova, Czech Republic, 4,835
7. Sara Errani, Italy, 4,435
8. Jelena Jankovic, Serbia, 4,230
9. Angelique Kerber, Germany, 4,070
10. Caroline Wozniacki, Denmark, 3,520