Q. (Re- question 2443 above) What is considered to having began chazaras hashatz in order to continue if one left and there is no more minyan? Does “Hashem seffasai tiftach” count?
A. Although Mishna Berurah (66: 35) maintains that saying Hashem Seffasai counts as the beginning of shemonesreh for certain purposes, such as not interrupting anymore with replying amen, for the issue of repeating aloud the amida, (see question 1387), when one left and there is no more minyan, Poskim offer different rules.
From the simple reading of the Shulchan Aruch (O.H. 55: 3), it would seem that just the first word “Boruch”of the first brocho counts (Piskei Teshuvos ibid. 8: n. 56). However, Eishel Avraham (Butchatch, ibid) rules that it means the first three words. (Boruch Ato Hashem).
Horav Shlomo Miller’s Shlit’a maintains regarding mincha on a ta’anis, that once one has started the amida and said the first two words of the bracha he should continue with the rest. (See question 1362 and 1387).
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a