Jankovic showed all her experience and guile in the last two sets

World number two Jelena Jankovic fought back from a set down to book her place in the last eight of the US Open at the expense of Dane Caroline Wozniacki.

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A sluggish Jankovic allowed Wozniacki to break early and seal the first set 6-3 but the Serb found her rhythm and came through 3-6 6-2 6-1.

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The second seed will meet Austria's Sybille Bammer in the quarter-finals.

Olympic champion Elena Dementieva needed just 61 minutes to see off Li Na of China 6-4 6-1.

The Russian, seeded fifth, will now meet Patty Schnyder.

Just as she had at Wimbledon earlier this year, Jankovic dropped the opening set against 18-year-old Wozniacki.

But once the second seed broke for a 4-2 lead in the second set, the Danish teenager's resistance crumbled.

"I had to really play a lot more aggressive because it's quite windy and she was a lot more solid than I was in the first set," said Jankovic.

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Jankovic is one of four players who can overtake compatriot Ana Ivanovic as world number one at the end of the tournament.

Russians Dementieva and Dinara Safina along with Serena Williams are the others.

But Jankovic said her main aim was to capture her first Grand Slam title.

"It (the number one ranking) is a goal, but I want to win a Grand Slam," she said.

"Of course, by winning a Grand Slam I will regain the number one ranking, and that is something I want really bad. I will try my best to achieve that."

Dementieva can become world number one if she reaches the final and the 26-year-old believes she is returning to the form which saw her win the Olympic title in Beijing.

"It took me a week before I started to feel the ball and play a little better than in the beginning of the tournament," she said.

"After playing four matches here, I feel pretty focused on this event."

Bammer, the 29th seed, reached the quarter-finals of a Grand Slam for the first time when she came through a marathon match against Marion Bartoli.

The Austrian prevailed 7-6 (7-3) 0-6 6-4 against the 11th seed after three hours and three minutes - matching the longest women's match at the US Open on record.

Bartoli struggled in the hot conditions and twice sought treatment from the trainer.

"It's a miracle I could manage three sets," said Bartoli, who said she was suffering from a stomach bug and low blood pressure.

Schnyder, playing in her 12th US Open, reached the quarter-finals for the first time in 10 years with a 4-6 6-3 6-3 win over Slovenia's Katarina Srebotnik - who knocked out world number four Svetlana Kuznetsova in the previous round.