Following a two-month break after his loss to Daniil Medvedev in the U.S. Open final — which ended his pursuit of a calendar-year Grand Slam — Djokovic returned this month and won the Paris Masters.
He's now attempting to match Roger Federer's record of six titles at the season-ending event for the top eight players.
The victory gave Djokovic an early lead atop the Green Group in round-robin play, with 2018 champion Stefanos Tsitsipas playing Andrey Rublev later.
Medvedev and Alexander Zverev won in the Red Group on Sunday as the tournament made its debut in Turin after 12 years in London.
After the match, Djokovic was presented with the trophy for finishing the year ranked No. 1, a feat he accomplished for a record seventh time — breaking a tie with childhood idol Pete Sampras.
In an on-court interview, Djokovic recounted how he fell in love with tennis watching Sampras when he was “4 or 5 years old” and asked his dad to buy him a tennis racket.
“Now I'm here and I've won this (trophy) seven times, one more than (Sampras) — incredible,” Djokovic told the crowd in Italian.
“The dream continues. I hope to play this great sport for a couple of years.”
Ruud, a first-time qualifier, broke serve in the opening game of the match when Djokovic fell to the court and dropped his racket as he changed direction to run down a lob. Djokovic managed to pick his racket back up and hit a weak reply but Ruud then quickly finished the point off.
A double-fault from Ruud handed Djokovic the break back and evened the first set at 3-3. Djokovic then began holding easily and putting pressure on Ruud's serve, enabling him to wear the Norwegian down during the tiebreaker.
Helped by fast conditions inside the Pala Alpitour, Djokovic won a near-perfect 20 of 21 points with his first serve in the opening set, and 35 of 39 throughout the match.
Djokovic served 10 aces and Rudd had nine.