MOSCOW, Russia - Rising Slovenian Tamara Zidansek scored the best win of her young career at the Moscow River Cup on Thursday, defeating No.2 seed Daria Kasatkina in the second round, 6-4, 2-6, 6-4.

The World No.90 has soared up the WTA rankings this season, winning her first WTA 125K event at the Croatia Bol Open after Roland Garros, and her statement victory moved moved her into the last eight of a tour-level event for the first time.

The 20-year-old trailed by a break three times in the final set as the World No.13 built a rally from a set down, but ultimately won the last three games of the match to seal the best win of her career to date. 

"Daria is one of the top players in the world, so coming into the match I knew I would have to play well," Zidansek said after the match. "I managed to do that today."

Having just played her first career match against a Top 50 player last week at the BRD Bucharest Open, where she fell to Mihaela Buzarnescu in the first round, Zidansek looked undaunted against the two-time Grand Slam quarterfinalist as she was the first to break serve in just the second game of the match.

A streak of five straight games against the server ended the opening set, as Zidansek was the last to hold in the opener to lead, 3-2.

The Russian was consistently in trouble on serve in the set's late stages, as fell behind 0-30 in each of her next three service games, and double faulted for the seventh time in the opener to hand over the set. 

However, Kasatkina would not go quietly on home soil, as she won the first four games of the second set to quickly get herself back in the match.

The Russian raised her first serve percentage over 30 points in the middle set, from 42% to 79%, and struck just one double fault as she largely cruised behind her delivery.

After building her early lead, the No.2 seed held on late, as she saved a break point in the set's final game to send the match to a decider. 

"In the third set, we were together on the scoreboard all the time," Zidansek said. "At 4-all, we played two good points and I was really lucky to win those two points, which gave me the momentum to go and finish the match."

There were a combined 18 breaks of serve in the match, with each player winning nine games on return, but it was the Russian who struggled more overall.

She struck twice the amount of double faults as Zidansek, hitting 10 in the match, and saved just six of the 15 break points she faced. 

The Slovenian will next face Germany's Laura Siegemund, who defeated Kateryna Kozlova, 6-0, 6-3 with a spot in the semifinals on the line.

“I think I had a really good start, I was very focused and aggressive, and she couldn’t find a way [into the match]. In the second set I knew she was going to step it up – she had no other choice – it was a really tight match from there, even if it was very clear on the scoreboard," the German said.

"I had to step it up and stay composed myself because she started to play very well as she had nothing to lose anymore. It’s been a long time since I had so many matches in consecutive days. But you know I take it as it comes, and my body feels pretty good – it does need a lot of work, but that’s normal [after coming back from injury].”