Defending champion Roger Federer [Images] lost a set for the first time at this year's Wimbledon [Images] on Saturday before stumbling through to the last 16 with a 6-2, 6-7, 6-1, 7-5 win over German Nicolas Kiefer.

The aberration occurred when the Swiss top seed tossed away a 4-1 lead to lose the second set tiebreak 7-5 against the 25th seed, who beat Federer on grass at Halle three years ago.

The favourite for the men's title won the third set easily but, looking out-of-sorts on a murky Centre Court, trailed 5-3 in the fourth before turning it on his head with two consecutive breaks and sealing victory with an ace.

Federer, who is seeking a hat-trick of Wimbledon titles and who has not lost on grass for over three years, will play Spanish former French Open champion Juan Carlos Ferrero in the fourth round on Monday. The Spaniard beat Germany's [Images] Florian Mayer 3-6, 6-2, 6-1, 6-1.

"That was definitely a test today... definitely," Federer said.

"I had the feeling I should've won in three sets but in the end I am happy to have won in four."

Venus Williams [Images] awaits sister Serena

Venus Williams awaits sister Serena in the fourth round after beating Daniela Hantuchova [Images] 7-5, 6-3.

The siblings will collide head-on for only the second time since their 2003 final if Serena knocks out fellow American Jill Craybas later on Centre Court.

Champion in 2000 and 2001, Venus has struggled for form and motivation this year but she produced some vintage power tennis to fend off a stiff challenge from the Slovak.

Leading by a set and 3-0, Venus was striding away to victory only to drop serve twice in succession.

Normal service was quickly restored, however, and the 14th seed rattled off the last three games to take her career record over Hantuchova to 7-0.

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