Becoming the first Belgian women ever to enter the world's top-10 rankings on Monday, an assertive Van Roost needed just 56 minutes to dispatch the faltering Russian 6-3, 6-1.
Van Roost showed only a momentary lapse in the first set, allowing Kournikova to break and go up 3-2. But the Belgian displayed the same determination which allowed her to defeat Hingis in Filderstadt last week, winning the next four games to take the set.
Struggling with her service, the 17-year-old Russian laboured in the second set to retain her opening serve, before losing her next three and gifting Van Roost the win.
"She was more nervous and under pressure," said Van Roost. "She's probably the most famous player at the moment, even more than Martina. With people around her all the time, being in every newspaper, being the the star, she has to win a tournament. She needs to prove she's deserving of her status."
The Russian, who fell out of the top 10 after tearing ligaments in her right thumb in a fall in Eastbourne, is ranked 12th in the world. She has not lost to anyone ranked outside the world's top 15 since the start of 1997.
"I was a bit nervous but I'm not feeling pressure from anybody," denied Kournikova, who regularly looked over at her coach, Pavel Slozil, after losing a point. "I was just getting uptight as the match went on. I just need to get back to where I was before I was injured."
Two more seeds made first round exits before top seed Marcelo Rios and No 4 seed Jan-Michael Gambill restored some order in the Singapore Open yesterday.
Following the exit of the fifth-seeded Australian Andrew Ilie on Monday, the No 8 seed, Ramon Delgado of Paraguay, suffered a similar fate, being beaten 6-4, 6-4 by the American Justin Gimelstob. Also unable to justify his position in the draw was the seventh seed, Paul Haarhuis of the Netherlands, who started brightly before losing 0-6, 6-3, 6-4 to the Dane Kenneth Carlsen.
In the evening,Gambill was quick to demolish his fellow American Jonathan Stark 6-1, 6-2. However, the day belonged to Rios as the Chilean former world No 1 used his power with devastating effect to overwhelm the Canadian Sebastian Lareau 6-2, 6-0.Reuse content