The world number one is looking for his sixth ATP Finals title, though his last success came in 2016, and he overcame his Norwegian opponent 7-6 (7-4) 6-2 on Tuesday (AEDT).
Ruud showed glimpses of the form that saw him become the first Norwegian to qualify for the season-ending tournament, but despite coming close to edging the first set, he never looked like upsetting the Serbian, who recently clinched the year-end number one ranking for a record seventh time.
It started well enough for the 22-year-old, breaking Djokovic in the first game and going 2-0 up before the crowd had settled.
His opponent soon recovered to break back, showing some of his trademark steel to come out on top in the longer rallies. Ruud was able to take the first set to a tie-break, but Djokovic secured it with a beautiful forehand-winner down the line.
Ruud pulled off some impressive winners himself on occasion, with some nice drop shots in particular, but he was rarely able to keep up with the number one seed's pace and power.
Djokovic won 20 of 21 first-serve points in the opening set, and his overall performance stepped up another notch in the second as he returned the favour to Ruud by breaking him in the first game.
The result was never in doubt from there, with Djokovic dominating on his serve again, only failing to get his first serve in once in the second set.
Ruud did not manage another break point after the opening game of the match and was forced to see his opponent comfortably serve out to open his tournament with a victory.