What is this fruit, anyone knows?

That is a "Common Guava" (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)

Update: Now that the OP added a photo of the inside, I think Cos Callis was right, and it's Guava, it definitely doesn't look like an apple from the inside.

It looks very much like a Pond Apple:

The form of the leaves and the stalk/stem(?) looks also very similar.

The fruit itself looks like a caimito: a sweet tropical fruit. However, the leaves in the picture are throwing me off a little because caimito leaves should look very much like magnolia leaves - the kind that are made into Christmas garlands. The leaves in the picture are the right shape, but I am missing the waxy dark green of the dorsal side and matted gold color on the plantar side feature.