Lleyton Hewitt saw his only rival for the end-of-year world number one ranking crash out of the Masters Cup - and then kept his own tournament hopes alive by beating Marat Safin.

Andre Agassi's earlier defeat at the hands of Juan Carlos Ferrero meant Hewitt could no longer be caught at the top of the world rankings.

Safin destroyed Hewitt in Paris a fortnight ago, but was unable to find that level of consistency in Shanghai.

"It's incredible," said Hewitt afterwards. "I knew I'd got number one which is fantastic.

"I was nervous in the locker room, but I came out here and fought my butt off to win."

But Hewitt still needs a favour from Carlos Moya if he is to progress in Shanghai.

If Moya beats Albert Costa on Friday, Hewitt will join him in the last four.

Should Costa win, Hewitt's exertions against Safin will prove in vain.

The Spaniards, Costa and Moya, are great friends, but with $120,000 up for grabs for each win, Moya has plenty of incentive to beat his pal.

Safin had lost both of his first two games in the tournament, but still had a mathematical chance of going through if he could beat Hewitt.

The Australian has been far from his best throughout the tournament, and against Safin he struggled to find any rhythm on his first serve.

But Safin has also been nowhere near the player who blew everyone away in the Paris Masters a couple of weeks ago, and Hewitt took the first set 6-4.

However Safin then found his form in the second set and took two breaks as Hewitt's serve totally deserted him.

When Safin broke the Australian to lead 2-0 in the third set it looked all over for the world number one.

But he broke straight back, and then crucially took his third break-point in the ninth game to serve for the match.