Jankovic sent packing

Unseeded Thai Tamarine, who turned 31 in May and is the oldest woman left in the draw, screeched with delight and sank to her knees after Jankovic hit a forehand long on the first of three match points.

The world number 60 took full advantage of another erratic display from Jankovic, who appeared to be moving freely despite suffering a knee injury in struggling to overcome third-round opponent Caroline Wozniacki on Saturday.

A break in the sixth game was enough for Tamarine to go on and clinch the opening set, and at 2-1 to the Thai in the second both players called for treatment.

Jankovic received attention to her troublesome left knee while Tamarine lay face down on court as a trainer probed an apparent back problem.

The stoppage made little difference to the course of the match and a dejected Jankovic had no answer to the scuttling Thai's consistency from the baseline.

Defeat leaves Jankovic, 23, still waiting to make her Grand Slam breakthrough. The Serb, who won mixed doubles at Wimbledon last year, has not gone beyond the fourth round in singles and has yet to reach the singles final of a Grand Slam.

Four-times champion Venus Williams swept into the quarter-finals with an impressive display of power tennis against Russian teenager Alisa Kleybanova.

Williams, firmly on course to meet her sister Serena in the final, won 6-3 6-4 on court two.

The number seven seed, often a slow starter, was razor-sharp from the beginning against the 18-year-old Muscovite, who was broken in the eighth game when she double-faulted three times.

The youngest player left in the tournmanent battled hard against the vastly more experienced Williams, 28, whose game has sharpened from match to match.

In the second set, Kleybanova fought a fine rearguard action. She survived two match points at 5-2, broke Williams's serve, held her own to love, then finally succumbed 6-4 to the title holder.

Polish teenager Agnieszka Radwanska completed the destruction of the top four seeds at Wimbledon when she beat Svetlana Kuznetsova 6-4 1-6 7-5 in the fourth round.

In addition to Jankovic, the women's draw has also lost world number one Ana Ivanovic and former champion Maria Sharapova.

Russian fourth seed Kuznetsova led 4-1 in the decider but could not sustain the momentum and bowed out when she dumped the ball into the net on Radwanska's second match point.

Radwanska, seeded 14, reached the quarter-finals of the grasscourt championships for the first time and will next face twice former champion Serena Williams.

Kuznetsova appeared to be heading for an early exit when she lumbered around during the first set before storming back in the second set to restore order.

The Russian, bidding to reach the last eight for the fourth time in six appearances at the All England Club, appeared to have overcome any danger of another upset when she broke early in the decider to steam ahead 3-1.

But Radwanska never lost belief and kept chipping away at Kuznetsova's resistance before she finally drew level at 4-4. Kuznetsova had chances to break in the ninth game but was unable to tuck away that final point.

Her 19-year-old opponent pounced two games later to nose ahead 6-5 after Kuznetsova scooped the ball wide on break point.

The Russian punched away a sliced backhand volley in the next game to save the first match point but that proved to be her last hurrah.

Twice former champion Serena produced her best display at Wimbledon this year to wallop Bethanie Mattek 6-3 6-3 in their all-American fourth-round match.

The sixth seed, seeking her first singles title here since 2003, was in ominous form, breaking world number 69 Mattek in the first and final games to take the first set.

In the second set the 26-year-old Williams again broke her unseeded opponent twice, in the first and third games, to race to victory in just over an hour.

China's Zheng Jie, who knocked out top seed Ivanovic, upset the form book again, defeating 15th seed Agnes Szavay 6-3 6-4 to reach the quarter-finals.

Wildcard Zheng, ranked 133rd in the world before the tournament after missing most of last season with an ankle injury, broke the 19-year-old Hungarian midway through the first set to take the early initiative.

Szavay wrestled it back to race to a 4-1 second set lead but the busy Zheng rattled off five games in a row to reach her first Grand Slam quarter-final and join Li Na as the only Chinese players to do so at Wimbledon.

There she will face Czech Nicole Vaidisova who beat Anna Chakvetadze 4-6 7-6 6-3.

The 19-year-old 18th seed had looked to be heading for the exit because of her over-hitting and shaky service games in the first set against the 21-year-old Russian, seeded eight.

The turning point came in the second-set tie break, which Vaidisova took to love. That seemed to give her new confidence and she quickly stormed to a 3-0 lead in the third set.

She served out the match, winning it when the Russian sent her return into the net, to set up a last-eight meeting with China's Zheng.

Fifth seed Elena Dementieva - the highest women's seed remaining in the draw - breezed into the quarter-finals with a 6-2 6-1 victory over Shahar Peer of Israel.

Despite making the greater number of unforced errors, the 26-year-old Russian - who made the last eight here for the first and only time two years ago - dominated from the start, breaking the 24th seed at either end of the first set.

Peer, clearly still feeling the effects of Saturday's draining three-hour-25-minute win over Dinara Safina, rallied admirably at times but was unable to carve out any real opportunities.

Dementieva, a beaten finalist in the French and US Opens in 2004, next faces compatriot Nadia Petrova after the 21st seed beat another Russian Alla Kudryavtseva 6-1 6-4.