In the history of the "World Champion" award by ITF since its inception in 1978, only in four years, the award was given to a player other than who finished #1 in the ATP rankings chart.
1. In 1978, Jimmy Connors was YE #1, but Borg got the award (Borg won RG and W, Jimmy won USO).
2. In 1982, JMac was YE #1, but Jimmy got the award. (Jimmy won W & USO, JMac none).
3. In 1989, Lendl was YE #1, but Becker got the award. (Lendl won AO, Becker won W & USO)
4. In 2013, Rafa was YE #1, but Novak got the award. (Rafa won RG & USO and Novak won AO).
Except in 2013, even though the award was given to a different person, that person had won more slams in that year. So, it was quite unusual.
BTW, if I have to pick one player only, I would give 2013 to Rafa. But, the point is that it is not an universal consensus and the two were very close that year. So, it is certainly appropriate to include 2013 a part of Novak's dominant stretch.