Defence witness Tom Wolmarans, a ballistics expert, has placed doubt over a key argument from the prosecution that Reeva Steenkamp was defending herself when four shots were fired through the bathroom door.
The state claims that Oscar Pistorius shot Ms Steenkamp on purpose as she tried to hide in the bathroom of the athlete's house.
Mr Wolmarans claims that Ms Steenkamp may well have been standing when the first shot was fired and that it could have taken her by surprise. He also claims that all four bullets hit the deceased, whereas the prosecution claim one bullet missed, ricocheted off the wall and injured her back.
Finally, the state claims that the fourth and final bullet fired by Mr Pistorius hit Ms Steenkamp's hand and head as she tried to protect herself.
Former police officer Mr Wolmarans claims that Ms Steenkamp was leaned forwards towards the door when the first bullet hit her head. The next hit her arm, the third hit her hand before the fourth hit her head as she fell backwards.
"The left hand cannot have been against her head" because there were no wounds and no brain tissue on the inside of her hand, he said.
The evidence presented by Mr Wolmarans suggests that the four bullets were fired in rapid succession and Ms Steenkamp would not have been able to react before the fatal bullet hit her head.
It raises doubts over the prosecutions claims that Ms Steenkamp barricaded herself in the bathroom in an attempt to protect herself.
Mr Pistorius denies intentionally killing Ms Steenkamp last year, claiming he mistook her for an intruder.
The double amputee faces life in prison if found guilty of murder.
The case has been adjourned until Monday May 12. The defence is expected to rest its case next week.