Defending champion David Nalbandian kept his Masters Cup hopes alive, sent Swiss Roger Federer into the semi-finals and knocked Andy Roddick out with a 6-2, 7-6 victory over the American in Shanghai on Thursday.

The Argentine, who improves to 1-2 in round robin matches, defused Roddick's serve to give himself a route into the last four if Federer beats Croatia's Ivan Ljubicic in the later Red Group match.

Under the complicated system to separate players with the same record in the round robin stage of the year-ending tournament, Ljubicic, who was 1-1 in his first two matches, must now beat world number one Federer to go through.

Fifth ranked Roddick, a shadow of the player who came close to upsetting Federer on Tuesday, joins Spain's Tommy Robredo in being eliminated from the tournament after defeat in the first two-set encounter in a tight Red Group.

American James Blake qualified for the last four from Gold Group on Wednesday evening, while world numbers two and three, Rafael Nadal of Spain and Russian Nikolay Davydenko, face off for the final slot in the semis on Friday.