WRAPUP 1-Tennis-Serena into semis, Wozniacki fights pain barrier

* Serena through to semi-finals

* Wozniacki wins but seized by cramp

* Zvonareva suffers nose bleed

By Martyn Herman

DOHA, Oct 29 (Reuters) - World number one Serena Williams marched into the semi-finals of the WTA Championships on Thursday with a comfortable win over Elena Dementieva while sister Venus kept her title hopes alive long after midnight.

Serena's 6-2 6-4 victory completed a 100 percent record for the American in Maroon Group and confirmed her as the first player through to the last four at the season-ending showpiece.

Venus, who suffered narrow defeats to Dementieva and Serena in her first two matches, beat Svetlana Kuznetsova 6-2 6-7 6-4 in a hard-hitting clash and must now sweat on Friday's results before knowing whether she will be in action at the weekend.

Feisty teenager Caroline Wozniacki remained undefeated on her debut at the $4.5 million Championships after beating Russian stand-in Vera Zvonareva in a White Group match that at times resembled a hospital casualty department.