London - Novak Djokovic made a flying start to his bid for a record fourth successive ATP Tour Finals title with a 6-1, 6-1 rout of Japan's Kei Nishikori, while Roger Federer cruised to a 6-4, 6-2 win against Tomas Berdych on Sunday.
Defending champion Djokovic and six-time winner Federer have dominated the prestigious season-ending event for the last decade, lifting the trophy eight times between them in the last nine years.
Djokovic arrived at London's O2 Arena hoping to win the title for a fifth time and in the process become the first player to lift the trophy four years in a row.
The world number one showed why he is the firm favourite to do just that, demolishing Nishikori in 65 minutes to record his 23rd successive win and his 79th victory in 84 matches this year.
With two more group ties remaining against Federer and Berdych, Djokovic's stroll by the banks of the River Thames was a significant statement of intent to his rivals.
"No question I played some of the best tennis I've played this year," Djokovic said.
"Undoubtedly it was an incredible performance that I was hoping I could have coming into this tournament, where I've had a great record in the last couple of years."
Nishikori added: "He played unbelievable tennis. I'm very ashamed with this score, but I still have two more matches."
Djokovic has dominated to such an extent over the last 11 months that the sport's big four, previously comprised of the Serb, Federer, Rafael Nadal and Andy Murray, has been reduced to a one-man show.
The 28-year-old has amassed 10 titles over the last 11 months, including the Australian and US Opens, Wimbledon and a record six Masters tournaments.
Since losing to Federer in the Cincinnati final in August, Djokovic has been unbeatable and Nishikori never came close to inflicting his first Tour Finals defeat in 15 matches as the Serb secured a 38th consecutive indoor match victory.
Meanwhile, Federer, the oldest Tour Finals qualifier since Andre Agassi in 2005, is undeniably in the twilight of his career, yet the 34-year-old is still good enough to compete for the sport's top prizes and is in the hunt for a semi-final berth once again after easing past Berdych in 69 minutes.
To cap an enjoyable day for Federer, he was presented with an award as the fans' favourite player in 2015 and he said: "I enjoy getting awards, especially the ones that are awarded by the fans.
"I have always sought perfection. I look forward to your support next year, and the year after that, and the year after that."
Only Djokovic has won more Tour-level titles than Federer's six this year and it was the brilliant Serb who denied the world number three when they met in the Wimbledon and US Open finals.
Federer has made a habit of producing landmark achievements at the Tour Finals and, in a testament to his astonishing longevity at the highest level, this was his record 14th straight appearance at an event he has won a record six times in nine final appearances.
Berdych initially seemed capable of causing an upset when a pair of Federer double faults contributed to the Swiss being broken in the second game.
But the 17-time Grand Slam winner brushed off that sluggish opening with an immediate response as he hit back in the next game when Berdych slipped on break point.
A hard-fought set swung Federer's way when an error-strewn service game from Berdych gifted him a decisive second break at 4-4.
With the first set wrapped up, Federer tightened his grip with another break in the first game of the second set.
That effectively ended Berdych's hopes and Federer cruised to the finish line with one more break.