You should have noticed that the key to the condition "x_n< x for all n" is that no member of the sequence is equal to x. For example, if {x_n}= {1/n} then x= sup {1/n}= 1 but {1/n} is a sequence converging to 0 so has no subsequence converging to 1. Now, the fact, that x is a sup but NOT a maximum should lead you to what abhishekkgp did.