Kei Nishikori rued his lacklustre first serve against an "unbelievable" Novak Djokovic as he was defeated 6-1 6-1 in their ATP World Tour Finals meeting.
The Japanese had a first serve percentage of 53 in the Group Stan Smith opener at the O2 Arena, falling to defeat to the world number one in just 65 minutes.
It was Djokovic's 15th successive victory in the competition and, while Nishikori conceded the Serbian played impressively, he was frustrated by aspects of his own game.
"Obviously I didn't do well on my first serve, so that caused a lot of pressure as he always had a good return," he said.
"I thought I didn't play badly - I had great points with groundstrokes. I think my serve was the key as in both sets I lost my first service game.
"That put on a little pressure to come back. I mean, he also played unbelievable tennis. I'm very ashamed with this score, but there are two more matches to go."
The eighth seed confessed that he did not feel he had any chance of stopping Djokovic from extending his winning streak to 23 matches.
"I couldn't do anything. I was defending all the time," Nishikori added.
"He also had great defence and made me run from side to side. I didn't feel I had any chance."
Nishikori will face Roger Federer and Tomas Berdych in his remaining group fixtures.