Federer has twice finished runner-up in Monte Carlo
Roger Federer pinpointed his serve as the key weakness following his surprise third-round defeat by Stanislas Wawrinka at the Monte Carlo Masters.
Federer lost 6-4 7-5 to his fellow Swiss, ensuring his wait for a first title since October goes on.
"Today definitely my serve wasn't working," he said.
"I haven't served my best throughout the season, so I have to make sure I get my serve back because I just don't hit the spots when I really want to."
He added: "Not that I expect 10 aces a set here on this type of heavy clay but I just think it's something I need to work on, make sure my timing gets right, my footwork gets right, and my forehand, that I use it the right way."
Federer, who married long-time girlfriend Mirka Vavrinec last week, accepted a wildcard after originally planning to skip Monte Carlo.
"I didn't have enough preparation before coming here, but that applies for everyone," he said. "It's not an excuse."
The 13-time Grand Slam champion has not won a title since October and changed his mind over playing in Monte Carlo as he looks to find some form ahead of the French Open.
Federer played well in his opening match against Andreas Seppi but made a succession of errors on Thursday, facing 14 break points on his way to defeat.
"My forehand was hurting me in the beginning," said the 27-year-old. "I was making a few too many errors. That kind of gave him the advantage. It was unfortunate, but I thought Stan played well.
"I came to see where I was. Now I know that I need to work at training to try to come back more strongly at Rome (27 April).
"I'm not too worried because I wasn't expecting much at this tournament. It's not a good result but I didn't expect to dominate everyone here. These two matches have given me information, rather than confidence."