The other three quarterfinals Thursday also ended in upsets.
Third-seeded Amanda Coetzer of South Africa was beaten, 3-6, 6-0, 6-3 by Anne Kremer of Luxembourg, and sixth-seeded Julie Halard-Decugis of France downed compatriot Nathalie Tauziat, the No. 2 seed, 6-4, 6-4.
Davenport has played only two grasscourt matches in preparing for next week's defense of her Wimbledon title, and only three matches total since injuring her back at the Italian Open five weeks ago.
She rarely looked comfortable against the persistent Van Roost and the windy conditions.
Van Roost, who also beat Davenport in the first round of the French Open, took a 3-0 lead in the first set. Davenport rallied to win the set, but was quickly in trouble in the second.
Van Roost took a 5-0 lead with the help of consistent ground strokes and won the set handily. The No. 5 seed from Belgium then had to resist two break points in the final set.
She produced the 11th of her 13 double-faults, giving Davenport an opportunity for a 3-1 lead, but Van Roost saved it with an overhead. A netted volley gave Davenport another chance at 4-3, but she put a forehand long to let Van Roost off the hook.
The breakthrough for Van Roost came in the next game when a net cord gave her break point. Davenport then put a backhand long and Van Roost served out for a hard-fought, 2-hour 5-minute victory.
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