UPDATE 4-Tennis-Jankovic out as Nadal and Williams go through

*Wins for top seeds Nadal and Serena Williams

*Dolko shocks Jankovic

*Murray has to sweat (Updates with later games)

By Simon Evans

MIAMI, March 28 (Reuters) - Number three seed Jelena Jankovic suffered a shock second round exit from the Sony Ericsson Open after losing 6-4 7-6 to Argentine Gisela Dulko.

World number ones Serena Williams and Rafael Nadal eased through and the surprise came in the final game of the round.

Dulko, ranked 35th in the world, produced an inspired display to prevail in a second set tiebreak against Serb Jankovic, who was clearly struggling with her game.

Jankovic looked to have turned the corner when she broke serve in the sixth game of the second set but Dulko broke back in the ninth and the set went to a dramatic tie-break.

The tall Serb was 5-2 up but let her rival back into the match with a double-fault and never recovered.

"I haven't been playing well at all. I've been losing to players because I'm just not at my level.

"I'm not the same player as I used to be. I've lost a couple of matches which for a player of my calibre it's not really acceptable and so I lost a little bit of confidence. That showed today in the match," she said. .

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Dulko, who enjoyed the backing of a large Argentine contingent in the crowd said the win was the biggest of her career.

"I feel great after this fight. I think that was the most important (win) for me because I was always down, first set, second set, and then in the tiebreak.

"I just hung in there, and the things were good for me at the end. I was really, really happy," she said.

Nadal's game was preceded by a flamenco dancer on the stadium court, but there was little need for fancy footwork from the Spaniard as he enjoyed a routine 6-2 6-2 win over Russian Teimuraz Gabashvili.

The Spaniard will now face Portugal's Frederico Gil in the next round.

Williams, searching for her sixth win at Key Biscayne, did not sparkle but still had little trouble in her 6-2 6-3 victory over compatriot Alexa Glatch.

Having dealt with the 19-year-old Californian, a wildcard entry, Williams will now play China's Peng Shuai, seeded 32nd, who beat Italian Mara Santangelo 7-5 6-3.

"I definitely think I could have played better. But I had a few little jitters going out there -- no one wants to go out in their first match," said Williams.

Serena's sister Venus got off to a slow start but found her form to defeat spirited Israeli Shahar Peer 6-3 6-3 and ensure a meeting with German Anna-Lena Groenefeld in the next round.

In the men's event, fourth-seed Andy Murray of Britain had to graft for his 4-6 6-3 6-2 win over Argentine Juan Monaco.

The Scot had his serve broken twice in the first set and then faced a 0-40 deficit in the first game of the second set, but fought back to hold his serve and then took a firm grip on the match.

"The only thing that matters really is winning," he said. "If I played great today and lost then I'm sure everybody would be pretty negative about it.

Murray will play Chile's Nicolas Massu, a three-set victor over American Mardy Fish, in the next round. (Editing by Nick Mulvenney/N.Ananthanarayanan; To query or comment on this story email sportsfeedback@thomsonreuters.com)