"Eighteen" — (shemoneh esreh) the central prayer in Jewish life and liturgy. It is also known as Amidah (Standing), because it is said standing; or simply Tefilah (Prayer), the prayer par excellence. The prayer originally consisted of eighteen benedictions, and thus the name "Eighteen." Its final version dates from about 90-100 A.D. when a nineteenth benediction was added. Every Jew is religiously obligated to pray the "Eighteen" daily; however, in times of emergency saying an abbreviated form of the prayer may fulfill this obligation.” Source: Jerusalem Perspective http://www.jerusalemperspective.com
Rabbis typically presented a condensed version to their disciples for situations in which they did not have the time or could not pray then entire Amidah. The Lord’s Prayer seems to be such a condensation.