Third-seeded Dominika Cibulkova of Slovakia withdrew because of illness, and fourth-seeded Sybille Bammer of Austria was beaten by Russia's Vera Dushevina, 6-4, 7-5.
Cibulkova, world No. 19, was replaced by lucky loser Ryko Fuda of Japan, who subsequently lost 6-3, 1-6, 6-3 to Russian Vitalia Diatchenko. Diatchenko will face India's Sania Mirza in the round of 16.
Dushevina's victory secured her a slot in the quarterfinals.
In an earlier second round match, fifth-seeded Peng Shuai of China reached the quarterfinals with a 7-6 (4), 6-0 victory over Galina Voskoboeva of Kazakhstan.
Peng, world ranked No. 35, took some time to come to grips with her opponent's game but found momentum in the latter part of the match, advancing to the last eight for the second time in this event at the beach city.
"I've never played her so the first set was a bit tough for me,'' said Peng, who reached the Pattaya semifinals in 2007.
In first round competition, eighth-seeded Magdalena Rybarikova of Slovakia survived a tough challenge from 38-year-old Kimiko Date Krumm of Japan to emerge with a 6-2, 4-6, 6-4 win after two hours, 13 minutes.
Krumm, a former world No. 4 who came back on the tour after a 12-year retirement, appeared to have leg problems in the final set, and was seen in tears after the match.
In other first round matches, sixth seeded Tamarine Tanasugarn pleased the home fans by defeating Akgul Amanmuradova of Uzbekistan, 6-2, 7-6(4) and seventh-seeded Shahar Peer of Israel beat Thailand's Noppawan Lertcheewakarn 6-1, 6-0.
Peng Shuai advances to 2nd round
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