Roger Federer and Rafael Nadal through at Qatar Open

Roger Federer on his way to victory in Qatar against Andreas Seppi
Federer has won 20 successive matches since defeat at the US Open

Defending champion Roger Federer beat Italian Andreas Seppi 6-3, 5-7, 6-4 to reach the Qatar Open semi-finals.

World number two Rafael Nadal is also through after a 6-4, 6-4 victory over Russian Mikhail Youzhny.

Federer now faces Frenchman Jo-Wilfried Tsonga, who he beat in the final of the ATP World Tour Finals but lost to in the last eight at Wimbledon.

"I am looking forward to playing him again. I hope it will be a good season for both of us," said Federer.

"He's always a nice guy to see play, and a nice guy off the court as well - so I hope we can live up to expectations."

Federer's victory was his 20th in succession since losing to Novak Djokovic in the US Open semi-finals.

World number six Tsonga, who beat Spain's Albert Ramos 6-2, 6-1, said: "Federer is the best player in the world, probably ever, and it will be difficult to play him.

"You have to be perfect. I have to play my best tennis. But I know I can do it. I have done it already."

Nadal will play fourth seed Gael Monfils in his semi after the Frenchman defeated Serbian Viktor Troicki 6-2, 6-3.