Prosecutors allege Dr Murray lied to paramedics about Jackson's health, and covered up his use of propofol.
Dr Murray pleads not guilty to the charge of involuntary manslaughter.
Mr Senneff, who was the first member of his paramedic team to enter Jackson's bedroom, said Dr Murray appeared to be agitated and was sweating profusely.
"I knew that we got there very, very quickly. It meant we'd have a good chance of restarting the heart if that was the issue," Mr Senneff said.
But Jackson's skin was cool to the touch and his eyes were open, suggesting his heart had been stopped for more than five minutes, the paramedic said.
"That did not add up to me," he said to the jury.
The paramedic said he made other observations in the 42 minutes he spent trying to revive Jackson, which did not seem consistent with the information Dr Murray was telling him.
He told the court he saw Dr Murray collecting items near Jackson's bed, after the body was taken down to the ambulance.
The doctor also spent some moments alone in the star's bedroom before travelling with the paramedics to the hospital, he added.
'Gross negligence' In the first week of the trial the jury has heard from Jackson's concert promoter, choreographer, personal assistant and security guards.
Prosecutors have scrutinised events during the moments after Jackson stopped breathing, in an attempt to show that the doctor was trying to hide Jackson's use of propofol.
They claim Dr Murray's "gross negligence" of the star while administering the powerful sedative caused Jackson's death.
Dr Murray's lawyers argue that Jackson self-administered the fatal dose of propofol while Dr Murray was out of the room.
If convicted, Dr Murray could face up to four years in prison and lose his license to practice medicine. The trial is expected to last about five weeks