Del Potro, the Argentine star who won the 2009 US Open, was limited to three events last year because of a right wrist injury. He has jumped from 257th to 51st in the rankings this year, winning the Delray Beach International last month and reaching the semifinals in San Jose, Memphis and Indian Wells.

"I'm still improving match by match," said del Potro, 18-5 this year. "It's a good signal for my future, for my comeback, and for recovering my game." He will face Philipp Kohlschreiber in the second round.

"I feel tired, but I'm really glad with my success between San Jose and today, and I have more energy to stay alive in this tournament," del Potro said. "I really like playing here in Miami. Many South American people come to see the tournament, so we feel really well."

In a night match, James Blake came back to edge US compatriot Michael Russell 3-6, 7-6 (7), 7-6 (3).

South African Kevin Anderson also advanced, beating Russia's Nikolay Davydenko 6-4, 6-3.

In women's first-round play, former world No. 1 Dinara Safina of Russia beat Australian Jelena Dokic 6-4, 6-4.

American Bethanie Mattek-Sands celebrated her 26th birthday with a 6-1, 6-4 victory over Sweden's Sofia Arvidsson, setting up a match against top-ranked Caroline Wozniacki.

Mattek Sands lost the one time she played the Dane, in the 2009 Italian Open.

"It's a good challenge for me and I like challenges," Mattek-Sands said. "You just got to go into the match knowing you're not going to get free points. For me, I'll stick to my same game plan. I'm going to play aggressive." Japan's Kimiko Date-Krumm, at 40 the oldest player in the women's singles draw, beat Caech Zuzana Ondraskova 6-4, 6-3.

Meanwhile, Kim Clijsters is planning to avoid Japan over radiation fears since an earthquake and tsunami crippled a nuclear plant there.

Clijsters told Belgian media that she wouldn't go to the WTA tournament in Tokyo in September nor to one in Beijing the following month. The WTA released a statement from Clijsters on Wednesday.

"Most importantly, my thoughts and sympathies are with the people in Japan," she said in the statement. "It's heart-wrenching to see what they're going through right now. Of course the health and safety of anyone traveling to a potentially impacted area is my top priority as well as the WTA's, and I know that the WTA will continue to monitor the situation." Radiation has seeped into vegetables, raw milk, the water supply and seawater since a magnitude-9 quake and tsunami hit the Fukushima Dai-ichi power plant nearly two weeks ago.

The World Health Organization said last week that the radiation was unlikely to spread internationally.

During an interview in English on Tuesday, Clijsters expressed sympathy for what Japan is going through.

However, she never mentioned in that interview feeling uncomfortable about traveling to the region.

"It's obviously very tough to sit here, especially when it just happened and we were in Indian Wells following everything on the news, and I'm sitting here playing tennis and those people are struggling to stay alive," Clijsters said. "You're very worried and you feel very sad for the people and you cannot imagine what they're feeling and going through.

"I spoke to (my former doubles partner) Ai Sugiyama and was asking her if she was OK," Clijsters added.

Japanese player Kimiko Date-Krumm, 40, who won her first round match on Wednesday at the Sony Ericsson Open, admits she's still shocked by what took place in Japan.

Date-Krumm's family is safe and she's started a relief fund to assist victims of the disaster.

Date-Krumm understands Clijsters' anxiety about going to Japan. She's hoping her country will be able to host the event.

"We still have time and, hopefully, we can fix everything and everybody can come to Japan," Date-Krumm said. "First safety. If it's still not safe I can't push. But if it's 100 percent safe I hope everybody will come and help Japan."

Roger Federer and Andy Roddick said Wednesday that they haven't given any thought to whether they will play in the Tokyo event in October.

Federer said, "It hasn't even crossed my mind yet. I will only think about that when the situation is right in front of me. But we'll get the green light or not from someone else. At the end of the day you always have to take your own decision, but I don't think it's going to be a problem by then, to be honest." Roddick said: "I would say it's a long ways away for me.

Obviously, if it's deemed safe, I'll go."