Misaki Doi of Japan knocked out fifth seed Jelena Jankovic in the quarter-finals of the Luxembourg Open on Friday.
Jankovic claimed the Hong Kong title last week, but Doi snatched the first set in a tie-break before going 2-0 up in the second.
Jankovic hit back with a break of her own on two separate occasions late in the set, but Doi was able to hit back with a break of her own on each occasion, setting up three break and match points in the 12th game before sealing a 7-6 (7-4) 7-5 victory.
She moves on to face Belgium's Alison Van Uytvanck, who also caused an upset on the day, knocking out Barbora Strycova 6-4 6-4.
The other semi-final will be contested by Switzerland's Stefanie Voegele and Mona Barthel of Germany.
Voegele ended the run of her compatriot Laura Siegemund with a 6-4 2-6 6-1 victory, while Barthel ousted Mirjana Lucic-Baroni 6-3 6-4.