Djokovic beat Marin Cilic 6-2 7-6 (7-4) in last year's final
Defending champion Novak Djokovic secured his place in the semi-finals of the China Open with a 6-3 6-2 victory over Frenchman Gilles Simon.
The world number two broke three times and defended all seven break points against him to record his fifth successive win against Simon.
He will now play qualifier John Isner after the giant American beat fourth seed Nikolay Davydenko 7-6 (7-4) 6-4.
Second seed Andy Murray plays later on Friday against Ivan Ljubicic.
It is the fourth time in his last five tournaments that Djokovic has reached the last four, and he was satisfied with his progress after concluding the win over Simon in an hour and 25 minutes.
"I'm very happy that my serve is getting better and better and hopefully, I'm going to use it more and more," the 23-year-old said.
He was rather less enamoured about the conditions in Beijing, however, and said: "The bad air obviously does irritate you a little bit, especially if you're playing against somebody that you expect to play long rallies with.
"We had so many long rallies and it's hard to recover when you don't have fresh air.
"A box of oxygen or something on the bench would be great, but we don't have it. I guess I will have to get used to it."
The ATP said they would look into the air quality concerns.
Djokovic beat Isner in their only previous encounter in March, but took more than four hours and six match points to defeat the 6ft 9in American in Davis Cup action.