Roger Federer has hailed the quality of world number one Novak Djokovic.
The Swiss player lost to the 25-year-old Serb in straight sets in the final of the ATP World Tour Finals on Monday.
"Maybe there's a bit of regret because I had the lead twice before him," BBC Sport quotes Federer as saying.
"At the end of the day that doesn't matter, you have to get over the finish line in the set and then obviously the match. He was better at that today.
"What he does well - even in defence he stays somewhat offensive. That, I think, is what separates him from the rest a little bit."
Federer won the ATP World Tour Finals in 2010 and 2011.
