NEW YORK, NY, USA -- Daria Kasatkina has gotten the better of French Open champion Jelena Ostapenko in 2017.

The unseeded Russian handily defeated the No.12 seed, 6-3, 6-2, and moved into the fourth round of a Grand Slam event for the first time in her career.

"Before I was losing everything in third round, so I was not expecting anything in the US Open," Kasatkina told the press, after she finished her late doubles match. "I was just playing, enjoying, and trying to show my game."

Ostapenko had beaten Kasatkina in their first two head-to-head encounters, but Kasatkina turned the tables in their most important prior match, the final of the Volvo Car Open in Charleston in April of this year. It was more of the same on Louis Armstrong Stadium on Saturday.

At the start of the match, though, it looked like it was going to be one-way traffic for Ostapenko. Kasatkina served first, and quickly lost her initial service game with a double fault on break point.

"In the beginning, it [the match] wasn’t that well," Kasatkina said to the press. "I was a bit nervous, and she started quite good. But I just keep my strategy, keep focused, and it paid off."

Ostapenko raced out to a double-break, 3-0 lead in the match. She opened the match by effectively hitting her powerful groundstrokes from both sides, which her opponents at the French Open became heartbreakingly familiar with earlier this summer.

But then came a turnaround in the match, which was sudden and overwhelming. Kasatkina started to dial back her unforced errors, and focused on trying to draw errors from the Latvian, particularly off of her fantastic slice backhand.

Kasatkina got back on serve at 3-2, as her strategy paid off and the unforced errors began to flow off of the racquet of Ostapenko. In a matter of minutes, she had won five straight games to serve for the set at 5-3.

Kasatkina faced two break points in that game, but Ostapenko lost them with more unforced errors, and then provided another forehand unforced error on Kasatkina's second set point. The solid play by the Russian -- eight winners and only seven unforced errors -- put her up a set to the good.

"I was 3-0 up, and I think I missed one ball, and somehow the game changed, like, went the other way," Ostapenko said in her post-match press conference. "I knew she's gonna make me play uncomfortable, and I was preparing for that. But unfortunately, I couldn't give my best today."

Kasatkina, the World No.38, raced out to a 5-0 lead in the second set, as the 12th seed still could not control her errors -- she would finish the match with 38. At this point, the Russian's streak had extended to 11 straight games won, and it seemed imminent that Ostapenko would receive a dreaded second-set bagel.

"In the second set, I start to feel good," claimed Kasatkina. "Everything goes the way I want." However, at 5-0 in the set, one game away from victory, the Russian also admitted that she "got a little bit tight."

It was here that Ostapenko added more margin to her shots, and though she blitzed through the French Open draw with her all-or-nothing tactics, she was helped in this match by more judicious play. She claimed her first game of the second set with a backhand winner off the net cord, and then held for 5-2 with her only ace of the match.

But it was Kasatkina who pulled it together at the end. A fabulous 30-shot rally on her second match point, which featured hard hitting off both wings by Ostapenko and a drop shot and a lob by Kasatkina, went the Russian's way when the French Open champion fired a backhand into the net.

Kasatkina will now move into the second week of a major for the first time in her career. She will take on another power hitter: qualifier Kaia Kanepi, who is currently ranked No.418, but has a career-high ranking of World No.15, and has five Grand Slam quarterfinal appearances to her credit.

Kasatkina is wary of Kanepi, but prepared. "Of course, it’s not going to be an easy match at all," she said. "She’s a very good player, she’s had very good results in the past, and she’s already beat good players here. Her style of tennis is very dangerous as well.  So I have to be ready."