The world’s No. 375 South African outplayed Radoslav Lukaev of Bulgaria, 6-1, 7-6 (11-9), to advance against qualifier Kim Dong-Hyun of Korea.
Kim scored a 6-2, 7-5 victory over Dmitri Mazur of Uzbekistan.
"I played well. I made good volleys and returned the ball well," said the 21-year-old native of Pretoria, winner of two $15,000 Futures in South Africa last June.
Lukaev, who ousted local wild card Joseph Victorino, put up a strong challenge in the second set, taking a 6-5 lead after holding his serve in the 11th game.
But De Voest, doubles champion in the 1998 Mitsubishi Lancer Junior Championships held here in Manila, fought back to force the tiebreak.
"It’s hot and humid and I was a little tight in the second set but I managed to win the tiebreak," said De Voest, who led 6-4. The Bulgarian threatened at 9-8 but De Voest won the next three games to wrap up the match in 82 minutes.
Fifth seed Dmitri Tomashevich of Uzbekistan and No. 9 Daniel Andersson of Sweden also gained quarterfinal slots in this event supported by the ITF Grand Slam Development Fund, PSC, Viva Mineral Water, Manila Midtown Hotel and official ball Wilson.
Tomashevich rallied past Indon qualifier Febi Widhiyanto, 3-6, 6-2, 6-2, to advance against Briton James Auckland, a 6-1, 6-3 winner over Taiwanese Wang Yeu-Tzuoo.