Defending champion Roger Federer was stunned by Japan's Kei Nishikori in the last 16 of the Mutua Madrid Open today.

Despite a second-set fightback, the second seed was too often below his best form and succumbed in an hour and 35 minutes to end his first tournament back after a two-month break.

Nishikori led by a break in the first set and, although Federer held serve at 3-5 to keep himself in the set, he fired a backhand into the stand at set point as Nishikori took a shock lead.

Federer responded in predictable style, by winning the second set 6-1, but Nishikori broke in the fourth game of the deciding set to nudge ahead once more.

He held to lead 4-1 and then 5-2 and, after a pair of unforced errors from Federer set up match point, the Japanese player completed the break to wrap up a 6-4 1-6 6-2 win.

Nishikori told Sky Sports 4 afterwards: "He started playing much better in the second set, the match was going his way, but I played a bit more aggressive and was playing well.

"I think it's the biggest win (yet) for me - especially on clay, it's not my favourite court. It's an amazing day for me."

The result set up a shock quarter-final against wild card Pablo Andujar, who progressed after fellow Spaniard Daniel Gimeno-Traver was forced to retire at 5-5 in the first set.

Fifth seed Rafael Nadal, in contrast to Federer, was scarcely troubled in beating Mikhail Youzhny 6-2 6-3.

Nadal broke the Russian's serve on five occasions in the 75-minute encounter, wrapping up victory with a neat cross-court clip after chasing down a Youzhny drop shot.

Sixth seed Tomas Berdych beat Kevin Anderson 7-6 (7/5) 7-5, while France's Jo-Wilfried Tsonga, seeded seventh, battled past Fernando Verdasco 4-6 6-3 6-2.

In the women's draw, the top two seeds Serena Williams and Maria Sharapova both progressed to the quarter-finals in straight sets.

Williams dropped only four games against 13th seed Maria Kirilenko, breaking the Russian's serve twice in each set to win 6-3 6-1 in an hour and five minutes.

Sharapova was made to work hard in the second set against unseeded German Sabine Lisicki before eventually breaking for the second time in the set, and the sixth time in the match, to complete a 6-2 7-5 win.