Thai hope Tamarine Tanasugarn and Israel's Shahar Peer also won through to the last eight.
No. 5-ranked Zvonareva was in trouble at times against Shedova but managed crucial breaks in the 10th game and again in the opening game of the second to win in 1 hour and 40 minutes.
"It was difficult coming off a Grand Slam and play a new tournament which has different conditions, especially here which is hot and has two courts next to each other. It's difficult to concentrate because of the noises,'' the Russian player said.
"My level of game is not there yet but I fought hard for every point and played clean service games and didn't give her a chance to break.''
Zvonareva will play Peng Shuai of China in the quarterfinals, with the Russian boasting a 4-0 record in their head-to-head meetings.
"She is an up-and-coming player,'' Zvonareva said. "She can be very dangerous if she keeps working hard like she is doing now. She has a few good wins. Every time we play, they've been tough three sets.''
Seventh-seeded Peer, the 2006 Pattaya champion, progressed to the quarterfinal with a 6-4, 6-1 win over Yuliya Beygelzimer of Ukraine in just 73 minutes. Peer will play Russia's Vera Dushevina in the last eight.
Tanasugarn, seeded sixth, beat Portugal's Neuza Silva 6-3, 6-2.
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Zvonareva through to Pattaya quarterfinals
