Andre Agassi claimed an explosive 7-6, 3-6, 6-4 win over David Nalbandian on Thursday, reaching the semi-finals of the ATP Masters Cup and a step closer to the title he last held 13 years ago.

Agassi, who has not claimed the Masters since his first visit to the year-end finale in 1990, will next face Germany's Rainer Schuettler on Saturday in a rematch of the Australian Open final for a chance to play for season's final prize.

The other semi-final will pit Red Group leader Wimbledon champion Roger Federer against either world number one Andy Roddick or Argentine Guillermo Coria, who clash on Friday to decide the last berth.

With second place in the Blue Group standings and a spot in the final four of the $3.65 million year-end event riding on the outcome, tempers flared on a chilly stadium court with an enraged Nalbandian accusing tournament officials of favouring Americans with their calls.

After losing a hotly-contested first set tie-break that featured two disputed calls and Agassi triumphing 12-10 on a double fault, Nalbandian engaged in heated exchange with umpire Enric Molina heightening the tension in match already tingling with excitement.

That outburst brought ATP supervisor Mark Darby onto the court, who tried to calm the fiery Argentine.

Much of Nalbandian's anger was directed at tournament owner and promoter Jim McIngvale, who invested $27 million in this event but has come under criticism in some corners for his rampant support of American players from his front row seat behind the baseline.

"They all think the same," Nalbandian said when asked how the players felt about McIngvale's cheerleading. "I think it's a lack of respect towards the players."

EARLY BREAK

By the time Nalbandian returned to action he had regained his composure, registering the early break on way to a 3-0 lead and levelling the match at one-set all.

Playing in his first event since a semi-final loss to Spain's Juan Carlos Ferrero at the U.S. Open, the 33-year-old Agassi has shaken off a bit more of the rust with each match -- all three-set marathons -- and in the third set the Australian Open champion sparkled, once again using his experience and iron will to clinch a place in his sixth year-end semi-final.

"Coming off two months with no competitive tennis, it gives me a lot of confidence to go forward from here," Agassi said, looking ahead to his semi-final against Schuettler.

"He was obviously playing well down there (Australia) to get to the final.

"I go out there every match expecting a dogfight, expecting him to play well, expecting him to play better.

"I'll make him play a great match to beat me, that's for sure."

Earlier in the day, Schuettler stunned Roddick 4-6, 7-7, 7-6 spoiling the American's coronation as the year-end world number one.

Having clinched the year-end ranking when Andre Agassi downed Spain's Juan Carlos Ferrero on Wednesday evening, Roddick lacked his usual intensity against Schuettler never finding top gear, committing 49 unforced errors and eight double faults in a two hour, 35 minute struggle.

The German's victory took the edge off of celebrations as Roddick was presented with the year-end trophy by former-U.S. president George Bush during a ceremony conducted on centre court following the match.

In the day's only other match, Coria swept past Spain's Carlos Moya 6-2, 6-3 to keep his hopes of advancing alive.