Kevin Anderson underlined his credentials as the man most likely to challenge Novak Djokovic at this year's ATP Finals with a brutal destruction of Kei Nishikori.

Japanese world number nine Nishikori was on cloud nine after stunning Roger Federer in their opening round-robin match on Sunday, but he was brought crashing back down to earth by 02 debutant Anderson.

Nishikori was staring down the barrel of the ultimate humiliation, and only the second 6-0 6-0 defeat in the history of the Finals, following a ruthless display by the South African.

Yet at 6-0 5-0, and with less than an hour on the clock, the 28-year-old finally managed to string four serves together to get on the scoreboard and avoid the dreaded 'double bagel'.

It only delayed the inevitable, though, as Anderson duly served out for a 6-0 6-1 victory in 63 hopelessly one-sided minutes.

And with the rest of the field, Djokovic aside, looking unconvincing to say the least so far this week, Anderson is rapidly emerging as the biggest danger to the Serbian world number one's hopes of a sixth London title.

"That's amongst the matches best I've played," said Anderson, who was beaten by Djokovic in this year's Wimbledon final.

"I got off to a really good start and I think I did a really good job constantly applying the pressure and not letting up.

"Sometimes it's tough if you have a very dominating set, you often see scorelines that momentum can change. Today, I kept at it the whole time.

"I was growing in confidence and he sort of probably was going a little the other way, which is tough given the situation."

Despite the crushing defeat Nishikori will still have a chance of qualifying for the semi-finals if he beats Dominic Thiem on Thursday.

"For me, it was one of the worst days in the year," he said. "He was playing well, serving well and returning well, too.

"He played good and it just wasn't my day today. I need to try to forget about today. Something was wrong, I've got to fix it tomorrow and try to be positive for the next one."