BEIJING (Reuters) – Novak Djokovic extended his winning streak at the China Open to 27 matches with a 6-2 6-2 demolition over big-serving American John Isner on Friday.

The world number one was in devastating form, conceding just three aces in a masterclass performance to reach the semi-finals in Beijing.

The Serbian is chasing his sixth title in Beijing and his seventh this year, which already includes the Australian Open, Wimbledon and U.S. Open championships.

His next opponent is Spain’s David Ferrer, who won the Malaysian Open last week. Ferrer eased into the semi-finals with a 6-3 6-1 victory over Taiwan’s Lu Yen-hsun.

Rafa Nadal had a much harder time in his quarter-final with American Jack Sock, before prevailing 3-6 6-4 6-3.

The Spaniard is still struggling to get back to his best but said he was pleased with the way he fought back.

“Obviously I finished the match playing better than what I started. This year I lost a lot of matches when I had an advantage,” Nadal said.

“So to have the chance to win a match when I start losing … is important for me.”

Nadal will face Fabio Fognini in the semi-finals after the Italian beat Pablo Cuevas 6-1 2-6 6-2. Nadal has lost three of his last four matches against Fognini, including at the U.S. Open when he blew a two-set lead in the third round.

“When you play against a great player, especially if you don’t play to your best, your chances are lower,” Nadal said.

“If I am able to play my best tomorrow, I am going to have my chances. If not, it is going to be tough. It’s simple. Sport is simple. The winner is the player who plays better. Fognini played better in the three matches that he beat me.”

In the women’s draw, Wimbledon finalist Garbine Muguruza beat Bethanie Mattek-Sands of the U.S. 6-1 7-5 to ease into the semi-finals, joining Ana Ivanovic, Agnieszka Radwanska and Timea Bacsinszky.

(Writing by Julian Linden in Singapore, editing by Pritha Sarkar)