Moscow - Anastasija Sevastova reached the Kremlin Cup semi-finals by beating Russian veteran Vera Zvonareva 4-6, 7-5, 6-3 on Thursday.
The Latvian said she was lucky not to lose the match after Zvonareva - once ranked second in the world but in her first tour quarter-final since 2012 - broke her serve to level the second set at 5-5.
"I started playing better and better and toward the end I was playing one of my best matches here in Moscow," said Sevastova, the highest-ranked player left in the draw at No. 11.
A US Open semi-finalist, Sevastova will next face Tunisian qualifier Ons Jabeur, who upset eighth-seeded Anett Kontaveit 7-5, 6-1.
Jabeur has won six straight matches, including qualifiers. She has reached her first WTA semi-finals.
Jabeur was the 2011 French Open junior champion but has struggled to live up to her early potential.
Nick Kyrgios withdrew from the men's tournament with an elbow injury, giving Mirza Basic a walkover to the quarter-finals, where he'll play Karen Khachanov.
Kyrgios said it was a recurrence of an injury which caused him to miss several tournaments around and including the French Open.
"I'm going to choose to wrap up my season, head home and start the recovery," the Australian said. "See you all in 2019."
Second-seeded Daniil Medvedev reached the quarter-finals with a 6-2, 6-1 win over Dusan Lajovic in just 51 minutes. He will next face last year's defeated finalist Ricardas Berankis.