On a cloudy Paris day, Djokovic breezed through the opening set after breaking immediately but it was not all plain sailing for the world number one on Court Philippe Chatrier as Nieminen broke to lead 3-1 in the second set.
The 33-year-old left-hander, appearing at his 50th major, found himself serving for the second set at 5-3 up only to blow his chance by losing the next four games.
After that, the final outcome was in little doubt and Djokovic secured victory on his fourth match point to make it 23 consecutive wins since losing to Roger Federer in the final in Dubai three months ago.
Djokovic, who is seeking to win the French Open title for the first time and become just the eighth player to claim a career Grand Slam, will now face either Luxembourg's Gilles Muller or Paolo Lorenzi of Italy in the second round.
He is on a collision course with reigning champion Rafael Nadal in the quarter-finals, with the Spaniard easing into the second round by beating French teenager Quentin Halys in straight sets a little earlier on Tuesday on the centre court.