Defoe scored his fourth Carling Cup this season

Darryl Flahavan was superb in the Southend goal, saving first-half shots from Steed Malbranque and Danny Murphy.

Mido also hit the bar for Spurs with a powerful volley in the first half with Flahavan only able to look on.

In normal time Mido was denied by another superb Flahavan save, but the Southend keeper could nothing about Defoe's 115th-minute close-range goal.

Interview: Tottenham manager Martin Jol

Interview: Southend manager Steve Tilson

The impressive Mido played an important part in the goal, providing the pass for Defoe, who had had a quiet game up to that point, but finished with aplomb.

Southend had reached the last eight of the competition after their thrilling win over Manchester United, courtesy of a stunning Freddy Eastwood free-kick.

Eastwood's reputation ensured he was given plenty of attention by the Spurs defence and he found it difficult to influence the game.

It was a defender - Efetobore Sodje - who had Southend's best chance, the Nigerian going close with a second-half header.

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Former Spurs player Mark Gower made a lively start for the visitors, sending in a dangerous cross from the left in the fourth minute.

Gower also forced Spurs goalkeeper Paul Robinson into a save at his near post at the other end.

Robinson's counterpart Flahavan was busier saving efforts from Benoit Assou-Ekotto and Steed Malbranque.

But midway through the first half Flahavan had a lucky escape as Spurs hit the bar through Mido.

After good work by Murphy and Defoe, the Egyptian striker powered a volley against the bar.

Murphy and Defoe had chances themselves with the former Liverpool midfielder going close with a flicked header, while Flahavan had to save a headed effort from the Spurs and England striker.

Early in the second half Flahavan was forced into another save as he dealt with a curling shot from Malbranque and a Murphy effort.

Defoe also glimpsed goal when Sodje misjudged a ball over the top but the England striker lobbed wide.

Sodje went close for Southend with a header, while in the last minute of stoppage time Mido came desperately close to winning the game for Spurs, only for his header to be superbly saved by Flahavan.

But with five minutes remainging, Defoe slid in to connect with Mido's pass.

It was a cruel conclusion for Southend and particularly for Flahavan, who had done so much to frustrate the Premier League team.

The draw for the semi-finals will be held on Saturday.

Tottenham: Robinson, Assou-Ekotto, Dawson, Tainio (Davids 56), Stalteri, Davenport, Malbranque, Huddlestone, Murphy (Berbatov 76), Defoe, Mido.
Subs Not Used: Cerny, Lee, Gardner.

Booked: Stalteri.

Goals: Defoe 115.

Southend: Flahavan, Hunt, Prior (Clarke 24), Sodje, McCormack (Francis 118), Campbell-Ryce (Hooper 114), Maher, Hammell, Gower, Eastwood, Bradbury.
Subs Not Used: Welch, Paynter.

Booked: McCormack, Hammell.

After Extra Time.

Att: 35,811.

Ref: P Dowd (Staffordshire).