LEAD: Del Potro building confidence, Wozniacki sent out in Sydney

Jan 11, 2012, 13:31 GMT

Sydney - Juan Del Potro reached the quarter-finals of the Sydney International on Wednesday with the Argentine making a statement of intent with his 6-4, 6-2 defeat of Pole Lukas Kubot.

Agniezska Radwanska beat her best friend to earn a first victory over a world number one with her 3-6, 7-5, 6-2 defeat of world number one Caroline Wozniacki for a women's semi-final spot.

The loss opens the door for number two Petra Kvitova to take the top ranking if she can lift the title.

Wozniacki served for a straight-set win but was broken, with Radwanska taking it to a deciding set after five set points. The Dane was treated for a left wrist injury and was unable to close the gap on the Pole.

'It was a great match for me,' said the winner. 'It was a very good match, Cara was playing good and I had trouble in second set. I'm happy I came back in the end.'

Kvitova did her part with a win over Daniela Hantuchova 6-0, 6-4 while third seed Victoria Azarenka's match was hit by a power failure before defeating Marion Bartoli 7-5, 6-4.

French Open winner Li Na, the fourth seed, moved ahead at the expense of Lucie Safarova 6-2, 7-6 (7-3).

After two seasons of wrist surgery recovery and a shoulder injury, Del Potro ranked 11th is ready to make the big push back into the elite, here he once stood fourth on the ATP.

But the South American is not pushing things, preferring to begins his season by winning one match at a time. 'It could be a big challenge for me to win a major this year, I'm still far from that,' he said. 'But I'm working hard and feeling good every day.

'I must take it step by step. To reach the second week of the Australian Open would be a good start to the year.

'I improved a lot last season and won matches. That helped me a lot with confidence, knowing that I can stay healthy. I don't think of my wrist now, that's a good signal for the future.'

American qualifier Bobby Reynolds put out second-seeded John Isner 3-6, 6-4, 6-3 as the big man struggled with his game. Number three Richard Gasquet, who played a weekend Hopman Cup final in Perth for France, crushed Italian Fabio Fognini 6-2, 6-1.

Number four Feliciano Lopez fell to Julien Benneteau of France 6-3, 6-4.

There was another upset as Finn Jarkko Nieminen outlasted eighth seed Radek Stepanek 7-6 (7-5), 5-7, 6-1. It was the second week in a row that the pair had met, with the veteran Czech winning their second-round match in Brisbane.

2010 Sydney champion Marcos Baghdatis kept up his winning pace, defeating Aussie Matthew Ebden 3-6, 7-5, 7-6 (7-3).

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