Serbia's Novak Djokovic hits a shot during his men's singles match against Argentina's Carlos Berlocq at the China Open tennis tournament in Beijing.

Beijing - Novak Djokovic was happy to reach the China Open quarter-finals in straight sets on Wednesday as he targets his fourth title of the season and presses his bid to be crowned year-end world number one.

The Serb, second in the world rankings behind Roger Federer, dismantled unseeded Argentine Carlos Berlocq 6-1, 6-3 to extend his unbeaten record at an event he won in 2009 and 2010 and missed last year through injury.

Djokovic is playing his first tournament since losing last month's US Open final to Andy Murray, and looked ring-rusty in the first round when he was taken to three sets by Germany's Michael Berrer.

But the five-time Grand Slam-winner, 25, won the first set against Berlocq in only 25 minutes and raced to a 5-0 lead in the second. He then lost three games carelessly and was relieved to wrap it up without playing a third set.

“It was an interesting couple of games at the end of the match where he came up with some good shots and we played some entertaining points,” Djokovic said.

“I should have done a better job in winning those games and winning this match earlier but in the end I won in straight sets. That's what matters the most.”

The top seed will play Austrian Jurgen Melzer in the next round. Melzer knocked out sixth seed Alexandr Dolgopolov in three sets.

Djokovic's main challenger for the title, third seed Jo-Wilfried Tsonga of France, enjoyed an easy passage when his opponent Nikolay Davydenko pulled out ahead of their match citing an injury.

In the women's draw, world number two Maria Sharapova coasted through her second-round match, while defending champion Agnieszka Radwanska came from behind to defeat Lourdes Dominguez Lino of Spain.

The third seed required physio treatment on her right thigh early in the match and looked to be heading out. However, she rallied to a 2-6, 6-1, 6-4 victory and will play either Li Na or Peng Shuai in the quarter-finals.

Russian tennis queen Sharapova made simpler progress as she beat Romania's Sorana Cirstea, ranked 29th, 6-2, 6-2.

“I thought I served really well today, much better than the first round,” the four-time Grand Slam champion said.

“She (Cirstea) is someone that likes to be aggressive, get the first shot and hit it really hard. I was happy I took her time away. It was important to try to take that away from her,” Sharapova added.

The 25-year-old will face Slovenia's Polona Hercog, 90th in the rankings, in the third round.

However, there was a shock in the last 16 when the 69th-ranked Romina Oprandi of Switzerland beat former world number one Ana Ivanovic in straight sets to progress to the quarter-finals.

Oprandi, who only got a place in the tournament after Serena Williams' late withdrawal, defeated the 11th seed from Serbia 6-4, 6-3.

“I really struggled to find my rhythm out there today, especially on the forehand,” said Ivanovic, the world number 12.

“The ball was coming without much pace and I wasn't really efficient moving forward and taking advantage of that,” she added. “It was the unforced errors that made a big difference.”

Ivanovic's compatriot Jelena Jankovic was also knocked out, losing to Spain's Carla Suarez Navarro 7-5, 6-4.

Navarro will play France's Marion Bartoli in the quarter-finals after the ninth seed overcame Julia Goerges of Germany 6-3, 7-6

(7/2). - Sapa-AFP