Juan Martin Del Potro withdrew from the Austrian Open today, suffering from an injury to his right toe.
The 20-year old from Argentina, who is seeded three, was due to play Germany's Philipp Kohlschrieber.
The injury has dealt a blow to his hopes of qualifying for the Masters Cup to be held next month in Shanghai, and comes during his first week ranked in the world's top 10.
Ranked 75 in April, Del Potro has become been one of the hottest players on the ATP Tour. Beginning in July, he has won 30 of 32 matches, playing five finals and claiming four titles.
Last week he reached the final in Tokyo, but was hindered by an upset stomach as he fell to Tomas Berdych.
The problem, a split toenail similar to the problem that recently bothered Jelena Jankovic in Stuttgart, began at the US Open, and he aggravated the injury during his quarter-final win over David Ferrer last week.
The nail then broke completely during his first round win over Austria's Martin Fischer on Wednesday. He also has a bruise on the big toe of his left foot.
His win against Fischer lifted him above James Blake into eighth position in the Race To Shanghai, but at the beginning of this week less than 100 points separated Blake and 22nd-placed Radek Stepanek.
There are just three weeks of the regular season to play, and Del Potro will hope that he will be fit to contest the Masters events in Madrid and Paris in a bid to solidify his position.
Del Potro has been told to rest the foot for the next few days, and he will now travel to Madrid for treatment in a bid to play there next week.
