NEW YORK -- Rafael Nadal wasn't too worried about playing on a hard court Monday night.
It was an exhibition against Juan Martin del Potro, his first chance to compete at Madison Square Garden.
But when the 11-time Grand Slam champion heads to the hard courts for an official match later this week at Indian Wells, he doesn't know how his balky knee will respond.
Just the fact that he's planning to play in California is encouraging for Nadal in his comeback from injury.
A week ago, he wasn't sure if his left knee could handle it. Then came his performance at the Mexican Open -- capped by a dominating victory over fellow Spaniard David Ferrer, the world's fourth-ranked player, in Saturday's final.
"I started to feel free to run to every ball," Nadal said at a news conference Monday morning, hours before the BNP Paribas Showdown. "That's fantastic for me."
Nadal later lost 7-6 (4), 6-4 to the 2009 U.S. Open champ, but had a wide smile on his face for most of the night. An exhibition may not be as demanding as an official match, but he clearly moved without fear of pain, leaping for overheads and crashing into the wall behind the baseline chasing down a ball.
Del Potro knows what it's like to return from a long layoff -- the Argentine missed eight months in 2010 because of a wrist injury.
"It's amazing how great he's recovered," he said of Nadal in an on-court interview after the match.
"He's going to be fighting for No. 1 very, very soon," del Potro added.
On one point, the two exchanged between-the-legs shots and headers that would have made countrymen Lionel Messi and Andres Iniesta proud.
In the exhibition tradition of bringing a celebrity from the stands onto the court, Nadal and actor Ben Stiller briefly played doubles against del Potro and a little girl from the crowd -- who proved to be a better volley-er than the actor.
In the first match, top-ranked Serena Williams beat No. 2 Victoria Azarenka 6-4, 6-3.
Nadal's first three tournaments back after missing seven months have been on clay -- the best surface even before the injury for the seven-time French Open champ. The hard courts are far more punishing on his body, and he has talked about perhaps playing on them less.
Still, he'd like to stick with the same full event schedule as in past years. Indian Wells, with an expected field that boasts the top men's players in the world, will help determine whether that is possible.
"That will be a big test for me," Nadal said. "Today I know I can play on clay; that's a very important thing to know for me."
This was the first event for Williams and Azarenka since they faced off in the Qatar Open final Feb. 17. Azarenka won that one 7-6 (6), 2-6, 6-3 to snap a 10-match losing streak against Williams, which included the U.S. Open final.
Both looked rusty Monday, with little of the electricity of their three-set thriller a few miles away at Flushing Meadows in September.
They finally started showing some shot-making in the eighth game, when Azarenka's lob landed on the baseline, drawing applause from the American. Williams, who hurt her ankle and back at the Australian Open, ran all the way into the barrier at the front of the stands to chase down a shot later in the game.
She raced back to the middle of the court, and Azarenka hit her overhead into the net. Williams plopped down to the ground for some rest.
Down triple break point in the fourth game of the second set, Williams suddenly regained her trademark big serve to rally back. Azarenka, who has yet to lose an official match this year while defending her Aussie title, was broken six times in 10 service games.
Facing break point at 3-3, she seemed to realize it was time to dispense with the serious tennis. Azarenka called her No. 1 fan, pop star Redfoo, onto the court to take over for a point. He showed good pace if not accuracy on his serve.
"He needs to practice before his exhibition," Azarenka joked later.
Williams and Azarenka also played a couple of rallies left-handed in honor of Nadal. They revealed they're planning to shoot a karaoke video together to Rihanna's "Stay."
Williams said Azarenka is one of the few players she's close to off the court.
"We really respect each other," Williams said. "I love Victoria as a person. I love how she's so competitive on the court."