Second-seeded Djokovic took just one hour, 29 minutes to claim the win, despite some inventive play at the net by the 27th ranked Hanescu.
A key moment came in the first set when, up 4-2, Djokovic saved two break points to hold serve and remain in control. The second set began with the players trading breaks to two games apiece. Each then held serve until Djokovic broke Hanescu to go up 6-5 and served out for the match.
"I was happy overall with the way I played. It's just that I thought I could finish the job earlier. It was a little bit frustrating the way I was losing some points in the second set," world No. 4-ranked Djokovic said. "That's the first round. You have to get used to the ups and downs a little bit and hopefully the next match will be more consistent and I'll be able to play my best," he said.
Among Hanescu's woes were six double faults, against three for his Serbian rival.
Djokovic said he hoped a day off Tuesday would allow him to overcome the effects of jet lag and acclimatize himself to the dry, sunny Beijing weather before facing the tougher competition ahead.
"I'm aware of that and I'm aware as well of the fact I need to raise level of performance in order to far in the tournament, so that's something that I'll work on .... you could see that in the second set I was a little bit sleepy," Djokovic said.
Pavlyuchenkova, who turned 18 in July, finished up with a 3-6, 6-1, 6-4 victory over Williams.
The American's match against the Russian who defeated her in the second round in Tokyo last week seemed set to go in Williams' favor as she took the first set 6-3. But she dropped the second and then scrambled to recover in the third to no avail.
Serving for the match, Pavlyuchenkova sent a decisive forehand winner past a reaching Williams to wrap up the final set after two hours, 13 minutes of play.
"She played really well. Sometimes I just made errors too soon in the point," said Williams, who totaled 12 double faults for the match.
Asked if there was something about the Russian's game that she found difficult to handle, Williams declined to analyze.
"She plays well against everyone she plays and I think I ususally do too," Williams said.
Pavlyuchenkova said she'd learned from her defeat to Williams earlier this year in Dubai that she had to take the game to her opponent, applying that to last week's match in Tokyo.
"I just tried to play as simple as I can just into the court and just make her move. And today I just tried to do the same thing," Pavlyuchenkova said.
"She started actually way aggressive today, especially the first set. She was playing much better, she was already preparing for this match more ... she wanted to kill me I guess," she said with a smile.
Top-ranked Dinara Safina, who could lose her No. 1 ranking to Serena Williams if she finishes behind the American at Beijing, plays China's Zhang Shuai.
Earlier Monday, Slovakia's Daniela Hantuchova ousted Spain's Carla Suarez Navarro 7-5, 6-2 and Italy's Francesca Schiavone got an easy route to the second round when Germany's Sabine Lisicki retired in the second set due to illness.
On Tuesday, No. 2 Rafael Nadal and defending China Open champion Andy Roddick start their campaigns in the men's draw.
The $4.5 million premier level event is being played at Beijing's National Tennis Center, the site of last year's Olympic tennis competition in which Nadal claimed the gold medal.