Roger Federer has admitted he can't put his finger on why he is struggling to recapture his dominant best form at the moment.

The world number one lost 6-4 6-7 (3-7) 7-6 (8-6) to Tomas Berdych of the Czech Republic in the fourth round of the Sony Ericsson Open in Miami to continue his recent troubles.

Federer started the year with another Grand Slam title at the Australian Open, but was surprisingly beaten in the third round of the recent Indian Wells tournament by Cypriot Marcus Baghdatis.

"I had tons of chances at the end. I tried everything I could. It is just not happening for me at the moment," he said.

"But I fought as much as I could under the circumstances with my game having issues at the moment."

Elsewhere, Rafael Nadal and Andy Roddick both advanced to the last eight following comfortable victories over David Ferrer and Benjamin Becker respectively.

Jo-Wilfried Tsonga, Mikhail Youzhny and Robin Soderling were also winners of their fourth-round matches.