WASHINGTON, DC, USA - No.7 seed Donna Vekic booked a championship date with former winner Svetlana Kuznetsova at the close of a marathon day at the Citi Open with a 7-5, 6-3 semifinal victory over Zheng Saisai.

The lone seed remaining in the draw among the four semifinalists, Vekic completed a pair of victories on Saturday to reach her first final of the season.

The Croatian No.1 first defeated Poland's Magda Linette in the last eight, in a match that wrapped up around 2 p.m., before finishing off the victory against Zheng as the clock struck midnight. 

"It's been a very tough day today...and I'm a little bit tired. I'm really happy that I managed to get through this one and in two sets," Vekic said after the match.

"I'm really happy with the way I played. It was a tough match, but I managed to keep my level."

Read more: Kuznetsova 'feeling great' as she sails into Washington final

The World No.44 stopped the Chinese No.3, runner-up at the Jiangxi Open in Nanchang last week, from reaching her second straight final behind 24 winners and six breaks of serve.

Vekic rallied from a break down in the opening set after losing the first two games to go a break up herself, but ultimately needed to win the last three games in succession to pocket a one-set lead.

After the two again traded breaks early in the second set, it was Vekic who steadied her way to the finish line in just over 90 minutes by winning four of the last five games.

"It's always great to be in a final," she said. "I don't think I've ever had to play two matches in one day and...last night, I was panicking quite a lot. My fitness is there and now I feel even more fit knowing that I can get through these matches."

Kuznetsova and Vekic have split two career meetings, but have not played in over two years. The Croatian's win in the head-to-head came in 2014 on the hard courts at the Miami Open.

"I'm good friends with her. We practiced here before the tournament and we practice quite often together," the No.7 seed said. "It's going to be a great match."