Roger Federer's bid for a first Grand Slam title gained momentum at Wimbledon on Friday when he beat American Mardy Fish 6-3, 6-1, 4-6, 6-1 to reach the last 16 of the men's singles.

"I'm happy to be in the second week," said Federer, who on this form could finally shake off the under-achiever tag that has followed him around the grand slams.

The 21-year-old Swiss, regarded by many as the natural successor to seven-times Wimbledon champion Pete Sampras, had Fish floundering on an overcast Centre Court as he rattled through the first two sets with near-perfect grasscourt tennis.

But a 40-minute rain delay at the end of the second set gave Fish time to recover his senses and he returned to give the fourth seed a real scare.

 From being in total control, Federer's serve suddenly lost its bite and he looked vulnerable as Fish began to display the form that took him to the Nottingham final last week.

Federer saved a break point at 3-3, but he was powerless two games later as Fish moved into a 5-4 lead after a superb rally that had both players scrambling around the court.

Fish served out the set and briefly threatened in the fourth, but Federer weathered the storm and broke for a 2-0 lead with a dipping, swerving forehand pass.

A double fault from Fish at 4-1 gifted Federer another break and this time there was no way back.

"I've got to keep the same attitude and intensity as I've had in the first week. I don't want to make the mistake of looking too far ahead."

Federer will play dangerous Spaniard Feliciano Lopez for a place in the quarter-finals -- the round he reached on his previous best showing at Wimbledon in 2001 after ending Pete Sampras' 31-match winning streak at the All England Club.