He admitted fatally hitting Sierra with a rock but disputed charges of sexual assault.
"I was sitting beneath the walls. I had sniffed paint thinner (vapors) and was high," Dogan quoted him as telling the court in Istanbul.
"I approached her and wanted to kiss her. But she hit me with her phone between my two eyebrows. I pushed her and she fell on the ground," he said.
Tasali said Sierra fought him back and hit him in the face with a rock, which made him "totally lose control." He said he then struck Sierra with a rock.
Questioned about traces of his sperm found over the victim and some of her belongings, Tasali denied rape.
A defense lawyer said he did not believe his client was "capable of grasping the meaning and results" of his actions. Judges hearing the case ordered state forensic medical experts to place Tasali under observation and report back on his mental health, the news agency said.
The court then adjourned the trial until Nov. 21.
Prosecutors are demanding a life sentence for Tasali on murder charges as well as 19 years in prison for sexual assault and theft.
Sierra had traveled to Turkey to explore her photography hobby. She came to Istanbul alone after her friend canceled for financial reasons.