Lleyton Hewitt celebrated his entry into the sporting hall of greats with a crucial win over Marat Safin at the season-ending Tennis Masters Cup in Shanghai on Thursday night.

Hewitt outlasted Safin 6-4 2-6 6-4 to leave Spaniard Albert Costa needing to upset his in-form countryman Carlos Moya in Friday's final Red Group pool match to deny the world No.1 a berth in the semi-finals.

Hewitt was assured of a second straight ATP Champions Race crown when Andre Agassi submitted 7-5 2-6 7-6 (8-6) in an epic battle with Spain's Juan Carlos Ferrero at The New Shanghai International Expo Centre.

Agassi courageously saved four match points before double-faulting trying to stave off a fifth, leaving Hewitt to become the first player since prolific American Pete Sampras (1993-1998) to successfully defend his year-end No.1 ranking.

Only five other players - Stefan Edberg (1990-91), Ivan Lendl (1985-87), John McEnroe (1981-84), Bjorn Borg (1979-80) and Jimmy Connors (1974-78) - have achieved the mighty feat.

Although he had secured back-to-back top rankings, Hewitt would have missed his chance to successfully defend his Masters Cup title had he lost the last set against Safin by more than two games.

So there was still much at stake when he took the court against the third-seeded Russian.

"I'm just over the moon at the moment to get this wonderful (Waterford Crystal) trophy again," Hewitt said after being presented with the spoils - including a bonus of more than $A1.8 million for winning last year's Champions Race and fulfilling his tournament commitments as world No.1 this season.

"I knew I had the No.1. I was very nervous in the locker room."

Swiss sixth seed Roger Federer clinched top spot in the Gold Group at the $A6.7 round-robin event after accounting for Czech Jiri Novak 6-0 4-6 6-2 and Ferrero beat Agassi.

Federer will play either Hewitt in the semi-finals, or Costa if the French Open champion downs Moya, who has already qualified for the weekend action.

In Saturday's other semi-final, Moya will meet the winner of Friday's final Gold Group clash between Ferrero and Novak, who both have one win and one loss so far this week.

The writing looked on the wall early tonight for Agassi, who had to win the tournament and hope Hewitt didn't reach the final to snatch the No.1 ranking.

The seven-times grand slam champion saved another three when trailing 6-3 in the tiebreak, one with a brave forehand winner, another with a Ferrero double, before the tension finally become too much to bear.

Agassi lamented not being able to convert his chances in the ninth game of the third set.

"That was certainly the biggest opportunities I had tonight," he said.

"He came up with some big serves that game. That was the game I had opportunities to squeeze the match.

"But it's going to boil down to a couple of points in a match that close."

Asked how disappointed he was to have double-faulted on match point with so much at stake, Agassi said: "That would be impossible. I wouldn't know how to communicate it."

A gracious Agassi said Hewitt was deserving of the No.1 ranking after another outstanding year in which the 21-year-old won four titles, including Wimbledon, the greatest prize in tennis.

"Lleyton has obviously had a great year," he said.

"He's again proven himself to be the best out there. It's not easy, especially the way he plays the game.

"All credit to him."

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