Johanna Konta is out of the Generali Ladies after a straight-sets defeat to Madison Brengle in Linz.
Konta, who at 47 is ranked five places below her opponent, was hoping to extend her excellent end-of-season form into another match after being handed a lucky loser spot in Austria and thrashing Annika Beck in the opening round.
However, fortune deserted her against Brengle, who claimed a 6-3, 7-5 win in 92 minutes to reach the last eight of the competition.
Nerves seemed to get the better of British No.1 Konta in the opening set, with a single break of serve in the fourth game enough to see her playing catch-up against her American opponent.
Brengle's dominance then faltered at the start of the second, with neither player able to hold serve across the opening four games while the American struck again in the fifth to take a 3-2 lead. This time Brengle managed to strengthen her advantage by holding serve, but Konta responded by reeling off the next two games to pull level and give herself a fighting chance.
The topsy-turvy contest continued as both players fought to gain a foothold until Brengle finally gained the decisive break in game 11 before serving out the match to set up a quarter-final meeting with either Sweden's Johanna Larsson or Belgian Alison Van Uytvanck.
Elsewhere, Caroline Wozniacki came from behind to beat Mirjana Lucic-Baroni 3-6, 7-6 (3), 6-4.
Wozniacki, who is seeded second, went up 2-0 in the opening set but won only two points in the next five games. She missed a set point at 5-4 in the second set but turned the match around in the tie-breaker.
Aleksandra Krunic provided the upset of the day, beating third-seed Roberta Vinci 6-1, 6-7 (3), 6-1. Krunic, ranked 99th in the world, had six aces and won 10 of the first 13 games.