MIAMI: Germany's Andrea Petkovic (above) pulled off another upset at the Miami Masters this morning, defeating sixth seed Jelena Jankovic to move into the semi-finals, while Kim Clisjers needed all her big-match experience just to stay in the tournament.

The 21st seed Petkovic, fresh from beating world No 1 Caroline Wozniacki on Monday, showed her exciting promise again with a 2-6, 6-2, 6-4 over Jankovic.

World number two Clijsters survived five match points to beat Ana Ivanovic, winning 7-6 3-6 7-6 to keep alive her title defense.

With some gutsy play from the baseline, the energetic Petkovic turned things around in the second set, saving four break points with her improving serve.

Jankovic fought back in the third, building a 4-2 lead, but Petkovic reeled off four games in a row to finish it, including a thrilling sevendeuce final game.

Petkovic will face the winner of the match between Russian Maria Sharapova and Romanian Alexandra Dulgheru, being played later in the morning.

While the 23-year-old Petkvovic celebrated her place in the final four with her trademark 'Petko Dance', Ivanovic was reduced to banging rackets after yet another reminder of her frequent inability to finish off matches.

The game, delayed by rain from Monday, was there for the taking at 5-1 when Ivanovic failed to convert four match points and she squandered another opportunity at 5-3 before losing the deciding tie-break.

Meanwhile, Rafa Nadal and Novak Djokovic both cruised into the quarterfinals of the men's event with straight sets victories.

Nadal faced virtually no resistance from Ukrainian Alexandr Dolgopolov in a 6-1, 6-2 victory, winning 86 per cent of first service points and did not face a single break point.

The Spanish world number one will face Czech Tomas Berdych, a 6-3, 2-6, 7-6 victor over German Florian Mayer, in the last eight.

Djokovic remains unbeaten this year, chalking up 21 successive wins, and that record was never in danger against fellow Serb Viktor Troicki in a straightforward 6-3, 6-2 win.

The world No 2 was comfortable throughout and will now face South African Kevin Anderson for a place in the semifinals.

Roger Federer was due to face Belgium's Olivier Rochus later this morning.

Federer and Nadal will face each other in the semifinals should the Swiss progress.

Argentina's Juan Martin Del Potro, still making his way back from lengthy injury problems, fell to American Mardy Fish 7-5 7-6.

Fish is now one win away from replacing Andy Roddick as the top ranked American after his hardfought victory in front of a crowd that was heavily backing his opponent.

Fish will overtake Roddick, who lost in his opening game here, should he beat Spain's David Ferrer in the last eight.

RESULTS

MEN'S FOURTH ROUND
Rafa Nadal (Spa) bt 21-Alexandr Dolgopolov (Ukr) 6-1, 6-2; Tomas Berdych (Cze) bt Florian Mayer (Germany) 6-3, 2-6, 7-6; Kevin Anderson (SA) btJohn Isner (US) 6-3, 7-6; Mardy Fish (US) bt Juan Martin Del Potro (Arg) 7-5, 7-6; David Ferrer (Spain) bt Marcel Granollers (Spa) 6-1, 6-2.

THIRD ROUND
Alexandr Dolgopolov (Ukr) bt jo-Wilfried Tsonga (Fra) 6-7, 6-4, 7-5.

WOMEN'S FOURTH ROUND
Kim Clijsters (Bel) bt Ana Ivanovic (Ser) 7-6, 3-6, 7-6

QUARTERFINALS
Andrea Petkovic (Ger) bt Jelena Jankovic (Ser) 2-6, 6-2, 6-4

SAN JOSE (Costa Rica): With Lionel Messi on the bench with an injury, Argentina looked lost and disorganised, settling with a dull 0-0 draw against Costa Rica this morning.

After a tough 1-1 draw against the United States last Saturday, the Argentines were hoping to rebound with a win over Costa Rica.

Argentina coach Sergio Batista made numerous changes from the squad who faced the United States, including resting Messi who had a minor injury and leaving Javier Zanetti and Angel Di Maria on the bench, but will be bitter with the result in front of a hostile environment in San Jose.

Argentina came close to scoring eight minutes in after Ever Banega played a stellar ball to the feet of Jose Sosa.

Banega took a touch in the box before shooting towards the far post with the outside of his foot, but goalkeeper Keilor Navas made a quality save to maintain the scoreline.

For the opening 15 minutes, the Costa Ricans hung tough defensively, playing physically while hardly giving Argentina room to advance with the ball.

A few minutes later, the hosts almost opened the scoring.

In the 18th minute, Bryan Ruiz nearly found the net with a low, driven shot from the left the box.

The shot almost went under Mariano Andujar, but the Argentina goal keeper was able to touch it wide. In the final 10 minutes of the first half, Costa Rica looked superior, keeping the Argentines on their heels.

With the momentum of the crowd in the Estadio Nacional, the Central Americans held possession in the Argentine half, forcing the likes of Gabriel Milito and Marcos Angeleri to clear the ball down field to relieve the pressure.

In the second half, Batista made a few changes, bringing on Mario Bolatti, Fernando Belluschi and Nicolas Otamendi.

Belluschi almost rewarded his coach with a goal when he fired a bullet to the right side of the goal that was touched wide by Navas.

But after that, neither team showed much of anything.

Both played horrendous, ill-timed passes out of bounds and failed to find the the crucial passes to send the forwards through.

In other friendlies this morning, Paraguay edged the United States 1-0 while Jamaica defeated El Salvador 3-2.