(Reuters) - Roger Federer said he is not playing well enough to have a chance of winning the French Open after the Swiss' preparations for the claycourt Grand Slam were cut short in Geneva on Tuesday.

The 39-year-old was up 4-2 in the final set against world number 75 Pablo Andujar of Spain but lost the next four games to lose 6-4 4-6 6-4 in his only tuneup event on clay before the French Open starts on May 30.

It was Federer's first match since his comeback in Doha in March after sitting out over a year due to two knee surgeries.

"I think when you played so little and you know where your level is at, and I think you saw that today, how can I think of winning the French Open?" the 20-times major winner told reporters.

"I'm just realistic that I know I will not win the French and whoever thought I would or could win it is wrong.

"Of course, crazier things might have happened, but I'm not so sure in the last 50 years at the French Open, somebody just rocked up at 40-years-old, being out for a year and a half and just go on to just win everything straight."

Federer said the next few weeks were important for his season.

"Regardless if you play tournaments or not, if you're back on the Tour or you're practising again with top guys that you go through the rhythm ... how it feels to be back on the Tour and this is then how you start taking better decisions.

"So when I walk out of a match like today, and I feel like 'My God, I could play so much better'. It feels strange and it's disappointing, but at the same time, this is the process I need to go through and that's why I can't get too down on myself and I need to go back to the drawing board."

(Reporting by Sudipto Ganguly in Berhampore, India; Editing by Peter Rutherford)