By Martyn Herman
LONDON (Reuters) - Novak Djokovic looked unstoppable when trouncing Kei Nishikori but for sheer swagger Roger Federer remains unrivalled as he demonstrated again when dismantling Tomas Berdych as the ATP World Tour Finals began on Sunday.
World number one Djokovic extended his latest winning sequence to 23 with a 6-1 6-1 hammering of world number eight Nishikori who was powerless to stop the rampant Serb.
Far from easing up with the finish of his best ever year a week away, Djokovic clearly wants to lay down a marker for next year's assault on the top silverware and showed Nishikori no mercy.
"No question, it felt like the best tennis I played this year," the 28-year-old, aiming for a fourth consecutive title at the O2 Arena, told reporters.
"I was at my best and it was an incredible performance."
Djokovic and Nishikori had stood heads bowed before their match to respect a minute's silence in a darkened arena for the victims of Friday's Paris attacks.
Following his immaculate performance he was presented with a trophy for the year-end number one ranking he has achieved for the fourth time in five seasons.
Home fans were given some early excitement when Jamie Murray, older brother of Andy, marked his debut at the tournament when he and Australian partner John Peers beat Italian duo Fabio Fognini and Simone Bolelli in a thriller.
But the atmosphere was subdued later as Djokovic snuffed out any chance of a shock when he thrashed a helpless Nishikori.
"He played unbelievable tennis. I'm very ashamed with this score," second-time qualifier Nishikori, who will hope for a better showing in his next two round robin matches, said.
Federer will provide Djokovic with a far sterner test when they meet in the "Stan Smith" Group on Tuesday, if his level for most of a 6-4 6-2 defeat of Berdych was anything to go by.
The 34-year-old, cheered on by a sizeable army of Swiss fans in various sorts of red and white attire, made a mess of his opening service game, falling 0-2 behind after serving two consecutive double-faults to gift Berdych a break.
But he responded with a break to love and then from 3-4 down in the opening set he produced a dazzling show reel of his favorite tricks to win seven games in a row -- throwing in a couple of his trademark half-volleyed returns for good measure.
"Was one of the worst starts I've had in years," Federer told reporters. "No first serves, two double-faults, nothing coming from the baseline. So it was rough.
"But thankfully I was able to pick it up."
Play begins in the Ilie Nastase Group on Monday with Andy Murray up against David Ferrer and Rafael Nadal taking on French Open champion Stanislas Wawrinka.
(editing by Justin Palmer)