Former world number one Jelena Jankovic has been warned not to under-estimate little-known American Melanie Oudin.

The 17-year-old qualifier, who is ranked 124th in the world, beat Yaroslava Shvedova of Kazakhstan in the previous round after knocking out 29th seed Sybille Bammer, of Austria, on her Wimbledon debut.

That has already earned her a place in the top 100 and victory over Jankovic today would take her another step towards a potential quarter-final with compatriot Venus Williams, who has already become aware of an exciting talent.

Oudin defeated Argentina's Betina Jozami on a memorable Fed Cup debut earlier this year, much to the admiration of the older Williams sister.

"I've played with her in Fed Cup and she's a really pleasant girl," said Venus. "She is very hard working and determined.

"She has a lot of talent and played great in the first Fed Cup tie."

Venus, meanwhile, will be expected to join sister Serena in the last 16 when she takes on 20-year-old Spaniard Carla Suarez Navarro, who she beat in three sets at this year's Australian Open, in the first match on Centre Court.

Elsewhere, top seed and world number one Dinara Safina plays good friend Kirsten Flipkens, the Belgian number two, for the first time, hoping to reach the second week of Wimbledon for the first time.

Svetlana Kuznetsova, the fifth seed, will be expected to become the fourth Russian to reach the fourth round when she meets unseeded German Sabine Lisicki for the first time in the second match on Court One.

Yesterday, three more seeds fell by the wayside in a bottom half of the draw increasingly dominated by two-time champion Serena Williams.

Eleventh seed Marion Bartoli, the runner-up in 2007, lost 7-6 6-0 to Italian Francesca Schiavone, 29, who had never previously been past the third round in eight attempts.

Russia's Elena Vesnina beat Slovakian Dominika Cibulkova, the 14th seed, while seventh seed Vera Zvonareva withdrew with an ankle injury, handing France's Virginie Razzano a walkover into Monday's fourth round.