Djokovic to meet Federer in the Rome Masters semi-finals

But Djokovic did not have it all his own way in the course of his one-hour and 23-minute victory, twice squandering a break advantage in the first set before stepping up in the second to eventually triumph 7-5 6-1.

Federer continued his fine clay-court form by blowing away the challenge of Italian Andreas Seppi in just 54 minutes in the late match.

Federer, the winner in Madrid last week, broke his opponent twice as he blasted through the first set and did not let up in the second, breaking twice more and serving confidently throughout to wrap up a 6-1 6-2 win.

His opponent Djokovic is seeking to retain the title he wrested from five-time winner Rafael Nadal last year, but the Spaniard fired a warning to his rival after seeing off in-form seventh seed Tomas Berdych 6-4 7-5.

Berdych, the losing finalist on Madrid's controversial blue clay last week, engaged the world number three in some furious baseline rallies and moved a break up in the second set before Nadal triumphed in two hours and six minutes.

Nadal told atptour.com: "I played one of my best matches on clay against a difficult opponent. It was a big match and one of the higher quality matches this year because the level of opponent is high and he is playing with confidence."

Nadal will face fellow Spaniard and long-time clay-court rival David Ferrer in the last four. Ferrer, who is aiming to reach his fifth tour final of the year, dispatched Andy Murray's conqueror Richard Gasquet 7-6 (7/4) 6-3.

Serena Williams is relishing the chance to put her perfect clay-court season to the test against reigning French Open champion Li Na after both players progressed to the women's semi-finals.

Ninth seed Williams spent only 28 minutes on court against Flavia Pennetta before the Italian was forced to retire with a right wrist injury, with Williams leading 4-0 in the first set.

Meanwhile eighth seed Li came through against Slovakia's Dominika Cibulkova, coasting the first set then holding her nerve in a tie-break to triumph 6-1 7-6 (7/4) in one and a half hours.

Williams told wta tour.com: "Li Na is the defending French Open champion. She's great to test yourself against because she's a good all-round player and she's so fit. But I've played on ice and water - I can play on any surface."

Williams' sister Venus was denied the chance to also move into the semi-finals as she fell 6-4 6-3 to second seed Maria Sharapova. The Russian served well throughout and saved two out of three break points en route to victory.

Sharapova will face Germany's world number 24 Angelique Kerber, who stunned fourth seed and Wimbledon champion Petra Kvitova after coming through a see-saw, two-and-a-quarter-hour contest to win 7-6 (7/1) 1-6 6-1.