What is this fruit, anyone knows?
That is a &quot;Common Guava&quot; (Wikipedia). When deployed to Saudi Arabia (years ago) we would get pallet loads of Guava Juice. My Puerto Rican friends were familiar with it already, it is supposed to be quite popular there (and throughout the Caribbean)
EDIT:
after looking at @arieljannai answer I felt a little more research was in order, I was unfamiliar with the pond apple and they do look a LOT alike on the outside. The leaf structure is even similar.
Britannica Has the following picture of a 'ripe' guava:

While this is the best picture of pond apple cut open:

Compared to the recently added photo of the original fruit provided by OP I am again confident that it is a guava (although, perhaps not a ripe one)