Top seed Belinda Bencic overcame a scare to make the last four of the St Petersburg Ladies Trophy on Friday, while Roberta Vinci showed her battling qualities to set up a semi-final with Ana Ivanovic.
Bencic lost a first-set tie-break to Anastasia Pavlyuchenkova, but showed character thereafter to prevail 6-7 (4-7) 6-4 6-2 in a match lasting more than two and a half hours.
Pavlyuchenkova delivered a somewhat mixed performance on serve, combining 11 aces with nine double faults.
Awaiting Bencic in the semi-finals is Daria Kasatkina, the unseeded Russian having also come from a set down to triumph 4-6 7-5 6-3 against Dominika Cibulkova - who knocked out third seed Caroline Wozniacki earlier this week.
Vinci, a finalist at last year's US Open, was involved in a thriller against Timea Babos, eventually winning through 7-6 (7-3) 4-6 7-6 (7-4).
Babos was leading 5-3 in the deciding set and 4-2 in the subsequent tie-break, but second seed Vinci fought back on both occasions.
Ivanovic was in fine fettle from the off in her 6-1 7-5 victory against Kateryna Kozlova, the Serb winning the first five games of the opening set.
Qualifier Kozlova staged a recovery from 5-3 down in the second, but it proved in vain as fourth seed Ivanovic powered into the last four.