Nadal also became the oldest player to end the year at the top spot since the ATP rankings were introduced in 1973.
Meanwhile, Muguruz, who dropped only one set en route to her first Wimbledon title and ended the year as world number two behind Romania's Simona Halep, lifted the women's award for the first time, the WTA reported.
It should be noted that it is for the first time since 1998 that the ITF world champions in both men's and women's category hail from the same country.
In the men's doubles, Polish player Lukasz Kubot and Brazil's Marcelo Melo crowned the world champions whereas US Open champions Chan Yung-Jan and Martina Hingis won the women's doubles award.
While it was a first ITF season-ending honour for Chan, Hingis clinched her third award in doubles (1997, 2015) to go along with three in singles (1997, 1999 and 2000). (ANI)