meaning in context - What does "if not" mean in the given sentence

Let there be given this sentence (which came from an English-Chinese dictionary):

The contest has become personalised, if not bitter.

Then what does the phrase if not mean?

Seeking after is a general guide or rule of such usage.

Answer

Let's look at simpler example -

Try to finish at least 10 chapters from that book, if not all.

This means if all chapters are not possible, try to finish at least 10.

That smell (from a rotten thing) can cause nausea, if not vomiting

This means that that smell is likely to cause vomiting but if it does not, at least it can causes nausea. In other words, that smell is capable at least to cause nausea but it can also go closer to vomiting or in worst cases, it can cause vomiting.

[Part A of sentence,] if not [part B of sentence].

In such cases, the part B is expected or desired but then actually part A is likely to happen.

I'll pick you at 1900 hr, if not earlier.

This simply means the latest will be 1900 hr. The speaker wants to say that he'll try to pick the listener earlier but not later than 7 pm.

Another such example may be - I'm a good tennis player, if not a great one.

So, in your sentence, the contest did not turn bitter but at least got personalized.

A later edit (from J.R. and user42307's input): The sentence may also mean that the contest is on the verge of getting bitter (nausea's example) or has become bitter.