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The Spanish world number two, chasing the biggest prize missing from his glittering resume, barely broke sweat as he outclassed the Russian debutant 6-3, 6-4 at the empty O2 Arena.
Earlier, US Open champion Thiem avenged his defeat by Tsitsipas in the championship match last year, beating the Greek sixth seed 7-6 (7/5), 4-6, 6-3.
Nadal, who last month equalled Roger Federer’s record of 20 Grand Slams by winning the French Open, has won 86 titles but just one of those has come on an indoor hard court.
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Nadal, 34, and Rublev both held serve comfortably in the early stages of their Group London 2020 round-robin match.
But the Spaniard, who was comprehensively outserving his 23-year-old opponent, made his move in the sixth game, breaking the Russian, who hurled his racket to the ground in frustration, and wrapping up the set with the minimum of fuss.
Rublev, who has won a tour-leading five titles during the coronavirus-interrupted 2020 season, was broken in the first game of the second set, leaving him with a mountain to climb.
The seventh seed upped his first-serve percentage but it was too little too late for the Russian, who failed to carve out a breakpoint opportunity.
“Always the first match is very tricky,” said Nadal. “Every match is difficult when you’re playing against the best of the best so it’s a positive start for me, very happy.”
“I think my serve worked very well tonight so that’s a bit of help always on these surfaces. I had my few chances on the return and I converted,” he added.
Nadal has qualified for the ATP Finals for a record 16 consecutive years but has been forced to withdraw on six occasions.
A combination of injury and his preference for other surfaces has counted against him but his greater freshness as a result of a shorter season may boost his chances this year.
In the opening match on Sunday, Austria’s Thiem missed a clutch of breakpoints in the first set against Tsitsipas but battled back from 4-1 down in the tie-breaker to take the lead.
The defending champion broke early in the second set to level the match and appeared to have the momentum but Thiem rallied impressively, earning a break with a delicately dinked forehand at the net in the second game of the decider, which proved decisive.
“I think the level was higher last year,” said Thiem. We were both in great shape last year in the final, we wanted that title 100 per cent. The atmosphere was insane last year in the final and today was a little bit different.
“Every win against a top-10 player here at the finals is something special and as well every win against Stefanos is something special,” he added.
Earlier, Novak Djokovic received the ATP Tour No. 1 trophy after finishing the year at the top of the rankings for a record-equalling sixth time.
Top seed Djokovic, who is competing in Group Tokyo 1970 alongside Daniil Medvedev, Alexander Zverev and Diego Schwartzman, plays his opening match on Monday.
The O2 Arena is hosting the ATP Finals for the 12th and final season before the year-end championships move to Turin next year.