The going gets tougher from here, says Rafael Nadal

As Wimbledon approaches second week, world No 1 is wary of threat from big guns.

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Rafael Nadal believes winning Wimbledon will be harder than ever this year because all of his closest rivals are fit and playing well.

Nadal secured a place in the second week of Wimbledon on Saturday by completing a 7-6, 7-6, 6-0 third-round victory win over Gilles Muller of Luxembourg.

Nadal, the defending champion and world No 1, will face Juan Martin del Potro in the last 16.

He overpowered Muller, but he knows far greater tests are ahead. Novak Djokovic remains in the tournament, as do old rival Roger Federer and Andy Murray. Those three are ranked Nos 2-4 in the world.

Nadal said the depth of talent in the men's draw made this year's Wimbledon one of the most unpredictable ever.

"You never know what's going on," Nadal said. "It seems like the best players are playing well. I don't know if I'm gonna win next round, so probably the rest of the players think the same.

"Every day is different. Every day anything can happen. Novak, Roger, Andy are playing well. But the matches are not easy to win. So the matches are close all the time. It's difficult to predict something."

Nadal, who is seeded to face Murray in the semi-finals, must first negotiate a way past Del Potro tomorrow, the Argentinian 24th seed who eliminated the 15th seed Gilles Simon in the third round.

"He's a very, very tough opponent. One of the best players in the world. He had a [serious] injury last year," Nadal said.

"His ranking for sure is much better than what the ranking (No 21) says today. His level is much better than what the ranking says.

"In my opinion, he's top-five level normally, if we talk about level. When he's healthy, he must be in the top five of the ranking normally. So will be a very tough opponent for me."

Federer was relieved to see off David Nalbandian 6-4, 6-2, 6-4 on Centre Court yesterday.

The Swiss third seed had lost to the Argentine eight times during his career.

"A guy who has beaten me eight times knows how to beat me again, so I am very pleased to go through," Federer said.

"I played well from start to finish. I am playing better than last year - more confident on serve and more relaxed with my returns. But things are going to get tougher."

In the biggest upset of the day, Australian Bernard Tomic defeated fifth seed Robin Soderling 6-1, 6-4, 7-5.

The 18-year-old qualifier said: "I had to play real good and it's probably the greatest achievement of my career. The crowd got behind me, which is good when you play a player like him. I may have looked very calm but inside I was bursting, I just tried not to show it."

Caroline Wozniacki, the women's top seed, produced a near-faultless performance as she eased into the fourth round with a 6-3, 6-2 victory over Australia's Jarmila Gajdosova.

The 20-year-old Dane made just five unforced errors in the match and will now face Slovakia's Dominika Cibulkova, the 25th seed. Cibulkova ousted the 16th seed Julia Goerges yesterday.

Serena Williams is also happy with her progress after defeating Maria Kirilenko 6-3, 6-2.

"I'm feeling better about my game," she said. "Every match is getting better.