Just a week after lifting the Paris Masters trophy, Novak Djokovic had his head bowed in a minute's silence for the victims of Friday's terrorist attacks in the French capital.
The world No.1 and Kei Nishikori paid tribute to the 129 people who lost their lives in Paris ahead of their opening encounter at the ATP World Tour Finals, which three-time reigning champion Djokovic took 6-1, 6-1 to open his title defence.
A large French flag was on display outside the venue on what was an emotional afternoon not just for the players but all in attendance. But there was also a visibly-increased security presence at the O2 Arena in the wake of Friday's events, with bomb-sniffer dogs and police vehicles on site.
Djokovic was presented on court with his fourth ATP World Tour No.1 trophy after the match, but that was clearly the last thing on his mind.
"I just would like to reflect on what happened a couple of days ago [in Paris]," said Djokovic. "All my heart goes to the families of the lost ones. It was really devastating to see those images and footages.
"I just want everybody to make one more applause in remembrance to these people."
It was a dominant display from Djokovic on his return to London. He broke Nishikori at the first opportunity on his way to taking the first set in 31 minutes, before wrapping up an emphatic victory in one hour and five minutes.
The records keep on being broken -- the win over Nishikori gives Djokovic a career-best tally of 79 wins in a single season. Winning the title here would also give him his best ever return (11) in one campaign.
The Serb is bidding to become the first player to win the World Tour Finals four times in a row in tournament history, and is seeking his fifth title here overall.
He extends his indoors winning streak to 38 matches, stretching back to 2012, with 15 victories in a row at O2 Arena.
It is also his 23rd consecutive win since the start of the US Open -- the longest streak coming into the season-ending event since Roger Federer's 24 wins in a row in 2006. Djokovic's last defeat came against the same man in the Cincinnati final on August 23.
"It has been the best season and year of my life," Djokovic said on court. "It did reflect in a very positive way my tennis game and career.
"I'm experiencing some of the peak time right now of the abilities and everything all coming together.
"I've had lots of success over the years but 2011 and 2015 definitely stand out. I'm looking back with a great pride and joy."
Djokovic, reflecting upon his incredible season, also reiterated that he almost quit the game in 2010.
"There is a reason why I've managed to reach the peak of my physical, mental abilities at this stage of my life," he added at his post-match press conference.
"I've had that great joy to win a first grand slam when I was 21 years old. Ever since then to this point, it's been seven years of a long process of learning, ups and downs, doubts, self-belief, some tough moments where I even considered of not playing tennis back in 2010.
"You go through these stages. Everybody does. It's just a normal, natural progress and process of development where I was fortunate to really have some great people who cared about me and who helped me to get to where I am today."