Shortly after reaching the third round at Wimbledon, defending champion Rafael Nadal called his next opponent "dangerous" and predicted it would be a "tough" match.

Sounds just like Rafa, the humble-talking Spaniard who always builds up the guy on the other side of the net.

This time, though, he may actually be right.

The 10-time Grand Slam champion will next face Gilles Muller of Luxembourg, the only man other than Roger Federer to beat Nadal at the All England Club since 2005.

"Was a long time (ago), but he beat me," said Nadal, the Wimbledon champion in 2008 and '10. "Gilles is a very dangerous player. He has a very good serve, good volley. He's especially very dangerous player in this kind of surface. So will be tough match for me."

Nadal advanced Wednesday by beating Ryan Sweeting of the United States 6-3, 6-2, 6-4. Muller had a much shorter match, going through when 31st-seeded Milos Raonic retired with a hip injury while leading 3-2 in the first set.

"Will be a big, big test for me," said Nadal, who missed the 2009 tournament with injury but is 16-0 at Wimbledon since losing to Federer in the 2007 final. "I have to be very focused all the time with my serve and try to convert the small chances that I can have on the return."

Six years ago, Muller defeated Nadal 6-4, 4-6, 6-3, 6-4 in the second round, only weeks after he won the first of his six French Open titles. Nadal is 28-2 at the All England Club since then, with both losses coming in the final against six-time champion Federer.

Although playing Muller at Wimbledon will not be new to Nadal, playing under the roof on Centre Court was a new challenge. The top-seeded Nadal was in the main stadium for the second straight match, but with rain early in the day the match went ahead with the top closed.

Nadal's take on the experience?

"The Wimbledon Centre Court with the roof or without the roof is probably the best court of the world," Nadal said. "Always a pleasure playing in this court."

But Nadal did say it was more humid than usual with the roof fastened shut.

"I don't know if (it was) because the atmosphere inside the court with the roof is more humid or because (it) was raining before the match or something," Nadal said. "But seems like the court is a little more slippery than usual."

Nadal made only seven unforced errors against Sweeting, although the 69th-ranked American managed to break once in the third set.

"I was playing well the first two sets, and especially the beginning of the third," Nadal said. "But 2-1, 40-Love, I had a few mistakes. And, yeah, he break me back."

If Nadal does make it through to another final, there is a good chance he could be up against Federer or second-ranked Novak Djokovic. But Nadal and his sheepish modesty won't let him get ahead of himself.

"Today my rival is every day my opponent and myself, too," Nadal said, "because I have to do well in every tournament in every match."