What pronunciation is '骑‘ in 千里走单骑’的‘骑’是什么的骑？qi 或者 ji?
《普通话异读词审音表》规定，“骑”在现代汉语中“统读”qí。

According to the 《The Authorized Table of Mandarin Words with Variant Pronunciations》，the word “骑”  is always pronounced with qí in modern Chinese. 

A famous and popular Chinese professor, 易中天 (Zhongtian Yi), who is famous for his lectures of 三国演义, pronounced "骑" in "千里走单骑" as qí. You may also search "千里走单骑" on youtube, and you may find that in many videos, it is pronounced as qí. For example, 0:38 in https://www.youtube.com/watch?v=PzLT9blEYdc