Roger Federer admitted his second-round exit to Tommy Haas at the Stuttgart Open was "not good enough" but he remains confident he can be successful this grass-court season.

The 18-time grand slam champion opted to miss the entire clay-court campaign - including the French Open - to give his body a chance to rest ahead the second half of 2017.

Federer's decision to focus on grass and hard courts looked to have backfired somewhat on Wednesday as Haas came from a set down to knock the number one seed out.

But Federer will not be pushing the panic button any time soon and remains confident he will be ready for the start of Wimbledon on July 3.

"Coming here and losing in the semis last year, now the opening round this year, it's not what I was hoping to do," he told the ATP website.

"Especially on grass courts, close to home, in Germany, which has been a good hunting ground for me. It's not good enough.

"If you don't take your chances like I didn't, leading a set and a break, you really only have yourself to blame at the end.

"I would have liked to stay here longer and given myself the best possible chance to win the tournament, so that's a let down."

The seven-time Wimbledon champion added: "It wasn't all bad, not at all. There were definitely some good moments, but I definitely was not as sharp as I was hoping to be in the big moments, or the moment when I had the lead and feel I should have been cruising.

"I definitely made some crucial mistakes and judgment errors.

"[But] it gives me more time for Halle. As a positive thinker, that's what I see. I've been on grass for over two weeks... it's good to play a match again.

"I really feel the body. It feels different right now than it does after practice. It just makes you tired.

"There are some positives to take away. It will give me good preparation going into Halle and then Wimbledon is soon."