Mardy Fish of the US serves to Argentina's Juan Monaco during the Paris Masters tennis tournament Nov 10, 2011. [Photo/Agencies]

PARIS - American Mardy Fish is facing a race against time to be fit for the ATP World Tour finals after pulling out of the Paris Masters due to a hamstring injury on Thursday.

The seventh-seeded Fish, who qualified for the Nov. 20-27 event in London thanks to Tomas Berdych reaching the quarter-finals at Bercy, was 6-1 6-7 2-1 up when he retired from his third-round match against Argentine Juan Monaco.

Fish had his left thigh massaged by the trainer after going 2-1 up in the decider but he could not resume.

"It's probably a little too early to tell right now. Pretty similar to what it was last week, just didn't get over the top," Fish told reporters, referring to the injury that forced him to pull out after just one game against James Blake in the first round of the Swiss Indoors tournament in Basel.

Fish, however, is willing to do whatever it takes to be ready for the season-ending tournament, the highlight of his 11-year career.

"It's just a pull. That's what the MRI said last week. I will get another one tomorrow here just to make sure," he said.

"We'll cover all our bases. So the next two weeks are arguably the most important of my whole career."

"I'll ice it five, six, times a day, electric stim it, ultrasound, all that stuff," Fish said.

There is, however, little chance that the 29-year-old will pull out of the London tournament.

"I've got a physio that travels with me everywhere. He knows my body better than I know my body," he said.  

"We'll be on it. We were on it 24 hours a day for the last, you know, 10, 12 days that I did it in Basel.

"I'll certainly play in London. There's no doubt about that, even if it's torn I'm still going to play and still try to play."