Pinki lost to a superior North Korean Mi Choi Pang, the latter winning 5:0.
What separated Mary Kom and Manisha was the patience that the 35-year-old employed during her bout and the aggression that Manisha displayed against Petrova.
With the crowd rooting for Mary, the tactical boxer didn't try to dominate to begin with biding time to inch her way, tackling the Chinese, a first-timer at the Worlds, before upping the ante in the second and third rounds for the unanimous decision.
Her lethal rights, when combined with the lefts and the jabs opened up the avenues for her to send down the blows on target areas.
In the semi-finals, Mary will meet North Korean Mi Hyang Kim, who won 5:0 against South Korean Chorong Bak. The Indian has a good record against Kim, having beaten the North Korean in the final of the Asian Championships last November.
"The World Championships are always tough. The Chinese come up with good new faces. I won't say it was very easy or very tough. I planned my strategy and fought accordingly," Mary said.
On her next opponent, the Manipuri said: "She could be coming with some plans against me because I had beaten her last year. But I am prepared for it."
Manisha, on the other hand, was full of aggression in contrast to the Astana silver medal winner's patience. In the first Manisha lagged behind her rival though midway she tried to catch up, trading combination punches.
However, the calculative Bulgarian went about methodically, landing a few lefts and a couple of straight on Manisha's face.
In the third, the former European champion was far superior reaching out to her Indian opponent which, perhaps, got the judges' nod, despite Manisha's last-round efforts.