¿Quieres un masaje?

¿Quieres un masaje?
 

¿Quieres un masaje?
¿Quiere un masaje?

These phrases are correct, right? ^_^

The first phrase is addressing someone in the tú form and the second phrase is addressing the question in the usted form, right? :)

These are questions, not phrases.

Yes, the first question begins with a verb conjugated in the second person, normally associated with the subject pronoun tú. You don't need to include the subject pronoun because the ending of the verb conveys the person.

The verb in the second question is conjugated in the third person. The third-person ending, however, can be associated with several subjects and subject pronouns, not just usted. So, unless it has already been established or can be deduced from context, you should include the subject/subject pronoun:

¿Quiere usted un masaje?
¿Quiere la señora un masaje?
¿Quiere ella un masaje?
¿Quiere el señor un masaje?
¿Quiere él un masaje?