ISTANBUL, Turkey - Fresh off Fed Cup heroics for France, Kristina Mladenovic scored a big first-round upset at the TEB BNP Paribas Istanbul Cup on Wednesday.

The Frenchwoman defeated No.4 seed Dayana Yastremska, 6-3, 6-4, to advance to the second round.

"It was not an easy match to start the tournament, and it's a tricky turnaround with Fed Cup around the corner," Mladenovic said. 

"We arrived pretty late for preparation but I was just trying to be more solid today, get my rhythm going. I was just more solid in the key moments today."

Mladenovic led Fance to its second Fed Cup final in four years alongside Caroline Garcia on Sunday, winning the decisive doubles rubber on home soil against Simona Halep and Monica Niculescu, and was back on court in the Turkish city less than 72 hours later.

The World No.66 broke the Ukrainian teenager three times in the opening set and three more times in the second set. 

She never trailed in the match overall, but needed to hold off a small charge from the World No.37 to seal the match in straight sets. 

Yastremska broke serve and won back-to-back games from 4-2 down in the second set to level at 4-all, but could not push the match to a final set.

"I really tried to absorb her game. She's really talented, explosive. She hits very big and you never know what to expect. You can get winners all over the place, and then some mistakes or not much rhythm," Mladenovic added.

"I'm just happy I served good overall, and just those small details. I managed to stay tough and composed to actually win this."

Arriving in Istanbul after a flight from France on Monday, and getting just one day of practice in on Tuesday, Mladenovic attributed her ability to ride the wave of emotions from her national heroics to Wednesday's big win.

"It was completely different conditions - we played clay courts, but indoors. There were emotions, with such a high adrenaline and big pressure," she added.

"It's such a nice feeling and a big event to reach a new Fed Cup final, but then, personally, you're physically and mentally tired. It was just about finding this rhythm in new conditions, new balls and all these things.

"It's a very nice feeling that we won last weekend - it's a big thing for us, for our country, but now this thing is done, and I want to focus on myself."

Another Fed Cup heroine also scored a first-round upset on Wednesday, as Czech Marketa Vondrousova defeated No.5 seed Ajla Tomljanovic in straight sets, 6-3, 6-3.

A pair of qualifiers in Ana Bogdan and Kateryna Kozlova advanced to the second round when their opponents retired due to injury, as Kirsten Flipkens and Anastasia Potapova retired in the second sets of each match while trailing by a set.