"I like the sunshine," Nadal said when asked about the difference of playing indoors and outdoors.
"The feeling on the ball, on your racket, is different" when playing indoors, Nadal said. "All of these small things that make you feel a little bit strange."
The conditions didn't seem to bother him Tuesday as the top-ranked Nadal beat fellow Spaniard David Ferrer 6-3, 6-2 on the indoor court at London's O2 arena.
Nadal came close to winning the event in 2010 when he lost in the final to Roger Federer. He didn't play last year because of a knee injury that kept him away for seven months before his successful comeback in February.
Nadal has won only one of his 60 titles on indoor courts. Bu he said he felt quite comfortable under the roof against Ferrer, who had beaten him in Paris last week in the semifinals.
"I'm happy. The court is in perfect condition here," Nadal said. "It's a good start for me, positive one."
The 13-time Grand Slam champion traded early breaks with Ferrer and then began dictating play from the baseline, forcing Ferrer into errors. Nadal converted six of seven break points, while Ferrer had 33 unforced errors.
"He arrived a little bit tired after playing the final (in Paris)," Nadal said. "I played with a little more calm than the other day (against him)."
Nadal, who will be guaranteed of the year-end No. 1 spot if he wins a second match this week, claimed his 21st win in 26 matches against Ferrer, but only his first indoors.
Nadal and Ferrer play in Group A, along with Stanislas Wawrinka, who beat Tomas Berdych in his opening match on Monday. No. 2 Novak Djokovic and Roger Federer, playing late Tuesday, are in Group B with Juan Martin del Potro and Richard Gasquet.