Some more examples so I hope it will be perfectly clear for you.

If someone says:
Я решил сложнейшую задачу.
- It does not necessarily mean "I have solved the most complicated problem". Depending on the situational context, it can be understood as "I have solved a very/extremely complicated problem". And most likely, it will be understood this way (as a very high degree of complexity).

However, if the same person specifies boundaries by saying:
Я решил сложнейшую в мире задачу.
- It becomes clear that he means "the most complicated problem in the world".

He can also say:
Я решил самую сложную задачу.
- This would always be understood as "the most complicated problem" (for example, he was given several math problems, and he has solved the most complicated of them) - the boundaries are understood from the situational context.

If he wants to specify boundaries, he can:
Я решил самую сложную задачу в мире (самую сложную задачу в учебнике, самую сложную задачу 20 века) etc.