ST. PETERSBURG, Russia -- Defending champion and No.4 seed Kristina Mladenovic of France kept her magic going at the St. Petersburg Ladies Trophy on Friday, defeating Czech Katerina Siniakova, 6-4, 6-3 to advance to the semifinals.

“I thought it was a very good match," Mladenovic said on court, after the match. "I think she’s a very good mover, and I think the first set was such high quality on the serve, from both of us. It was very difficult to have opportunities to break each other."

"I’m just very happy with the intensity I put in again today," the Australian Open women's doubles co-champion elaborated. "She has such great form since the beginning of the season, and she’s a tough opponent, so I’m very happy to be back in the semifinal here.”

The quarterfinal encounter became the Frenchwoman's seventh straight win in St. Petersburg, and she overcame a losing record against Siniakova, where she had previously been defeated twice by the Czech, versus only one prior victory.

In the semifinals, Mladenovic will face No.8 seed Daria Kasatkina of Russia, who upset World No.1 and reigning Australian Open champion Caroline Wozniacki of Denmark, 7-6(2), 6-3.

“Since the very beginning of the tournament, it’s only been a very high level here, very tough opponents," Mladenovic continued. "No matter who I play, it will be extremely difficult.”

It was a truly exceptional day on serve for the World No.10, who is trying to maintain her Top 10 singles ranking on the WTA, and needs to defend her title to have a shot at doing so. Mladenovic won 78 percent of points on her first serve and had seven aces.

"Katerina, she’s a very dangerous opponent, and she has great form since the beginning of the season," Mladenovic told the press after her match. "If you don’t play aggressive, she will take advantage, and then you’re the one running a lot, and I didn’t want to accept that."

As Mladenovic noted, Siniakova also served well, winning 69 percent of points on her first serve and dropping six aces, but the Czech was undone by 31 unforced errors, compared to only 12 off of the Frenchwoman's racquet.

It was in the first set where Mladenovic's prowess on serve really shone, powering her through a challenging opening stretch against the game Siniakova. Mladenovic won 93 percent of her first service points in the set, and never faced a break point.

Mladenovic held her first three service games without the loss of a point, but could not make inroads on the Czech's service, as Siniakova was holding her own with clean hitting, particularly near the net.

Eventually, Mladenovic took advantage of her pristine serving in the final game of the set, when a Siniakova slice backhand on game point went into the net. Mladenovic grabbed the unexpected deuce with vigor, striking a forehand winner to reach break point, and walloping a backhand pass to claim the maiden break of the match, and the opening set along with it.

A handful of scintillating passing shots from Siniakova allowed her to break Mladenovic in the first game of the second set, and the Czech continued to move forward more frequently than she had in the first set, trying to reverse the result.

But a screaming forehand winner square on the baseline by the Frenchwoman let her break back for 2-2, part of a run of five games which gave the fourth seed a commanding 5-2 lead, putting her a single game away from another semifinal appearance in St. Petersburg.

With Mladenovic serving for the match at 5-3, Siniakova hit an excellent forehand volley on the first point of the game, giving her a brief hope to extend the match. But it was only a minor speedbump, as Mladenovic took the final four points of the tilt en route to a straight-set win in 78 minutes.

"[Katerina] hits the ball very hard, very flat, so I’m just very happy the way I stayed in there, and won," Mladenovic concluded. "I was really satisfied with the pace of my shots in general, and I thought, from both sides, we played really fast. I think it was a really great match."