Djokovic attracted wild applause from the Turin crowd when he insisted on speaking in Italian for his on-court interview after the match, before which he was honoured once again for being the world's top player.
"It's a great feeling being back here in Italy... thanks for all your support over these years, Italy is like a second home for me," Djokovic said.
"It's one of the most difficult things to do in our sport, to finish number one at the end of the year you have to play really well, consistently and with a lot of success throughout the year on every surface. I'm very proud."
Norwegian Ruud, playing his first ever match at the Finals against one of the game's greatest ever, played creditably and could easily have taken a one-set lead.
The 22-year-old came out firing by breaking Djokovic in the very first game, but by game seven the Serb was in front 4-3 and looked well placed to take the first set.
Djokovic arrived at game 10 with his one-game lead intact and the match apparently turning in his favour but Ruud saved two break points to hold his serve and eventually take the set to a tie-break.
Ruud held a 4-3 advantage in the tie-break but Djokovic turned up the pressure by winning four straight games to claim the first set, and from there he raced clear.
Djokovic immediately broke Ruud at the start of the second set and from there he strolled to victory, breaking one more time on his way to claiming the second set and the match.
Rublev sees off Tsitsipas
Later, Russian Rublev impressed with a 6-4, 6-4 victory over 2019 champion Tsitsipas and sits behind Djokovic in the standings by virtue of winning one game fewer than the 20-time Grand Slam champion.
The fifth seed finished bottom of his group on his Finals debut last year but is now in a strong position to make the semi-finals ahead of his match with Djokovic on Wednesday.
"There's not much to say, he's one of the greatest players in history," said Rublev.
"I hope I'm going to win a couple of games. The only thing I can do is do my best and believe in myself."
It was an excellent return to form for Rublev after surprise early defeats in Paris, Moscow and at Indian Wells in recent weeks.
But the loss continued Tsitsipas' poor run. The world number four, who has been struggling with an elbow injury, has not reached a final since losing a five-set thriller to Djokovic in the French Open showpiece in June.
Rublev smashed nine aces and 27 winners in a dominant display, with breaks in the fifth games of each set enough to secure the win.