Rather than a crunch test of nerve, tennis ability and athletic prowess, this year's season-ending Masters Cup will be more the survival of the fittest after a gruelling 2005 on the men's tour.

Runaway world number one Roger Federer [Images] is once again favourite to retain his singles title when the $4.45-million event gets underway in Shanghai on Sunday.

To win five matches in seven days, however, the Swiss will have to be fully recovered from the ankle injury which put him on crutches just three weeks ago.

He says he is.

"A few weeks ago I was not sure how quickly my ankle would recover but now I am on the court and playing again," the Wimbledon and U.S. Open champion said.

To merely turn up in Shanghai represents something of a triumph for Federer compared to a number of his leading contemporaries.

Australian Open champion Marat Safin [Images] gave up his place in the eight-man field last month as he continued to struggle with a knee injury.

BACK SPASMS

The Russian has since been joined on the sidelines by world number three Andy Roddick [Images]. "Though I have continued to practise, train and play, the lower back spasms continue," the American said on Tuesday.

"After being evaluated by medical staff...it is now clear that I am not able to resume tennis activities for 10-14 days."

Roddick's withdrawal was a further blow to organisers still reeling from the withdrawal of twice-champion Lleyton Hewitt [Images].

The Australian cited personal reasons for pulling out -- his wife, television actress Bec Cartwright, is expecting the couple's baby in Sydney within the next two weeks -- but he has also been recovering from a groin strain and toe surgery.

Andre Agassi [Images] is one heavyweight contender to have answered the call and will fancy his chances of adding to his 1990 Masters crown.

However, the 35-year-old American has not played a match in two months, since the U.S. Open final, and continues to struggle with a sciatic nerve problem.

World number two Rafael Nadal's [Images] fitness is also something of an unknown quantity, after he sat out the final two weeks of the season to rest his knees because of tendinitis.

Argentine Guillermo Coria thanked his medical staff for helping him to secure his place in the field, while Russian Nikolay Davydenko has a sore serving shoulder and Ivan Ljubicic of Croatia played the Paris final last weekend with strapping on his right knee.

Only Argentines David Nalbandian and Gaston Gaudio -- late replacements for Hewitt and Roddick -- head into Shanghai with anything approaching a clean bill of health.

That alone could well be enough to see them triumph.

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