WUHAN, China - Czech qualifier Katerina Siniakova scored her second major upset in as many days at the Dongfeng Motor Wuhan Open on Wednesday, knocking out former runner-up Garbiñe Muguruza in straight sets, 7-6(3), 7-6(1).

Siniakova, who upset defending champion Caroline Garcia in the second round, pulled off another stunning escape to reach the biggest singles quarterfinal of her career and knock out the highest seed remaining in the field. 

Read more: Siniakova sinks defending champ Garcia in Wuhan marathon

The two-time major champion in doubles rallied from a 5-1 deficit in the opening set, one which Muguruza served for three times, before winning it in a tiebreak.

Siniakova also rallied from a break down in the second set, as she lost four straight games after holding a 2-0 lead, but nonetheless stood tall and sealed the win in two sets. The Czech ran her winning streak in tiebreaks this week to four, as she also won one in each of her first two matches. 

"I'm really happy about this, I'm just trying to keep working and now I hope it's showing," Siniakova said after the match. 

"I think I played more tiebreaks here than in all my life! I just tried to focus on every point...I just tried to keep fighting. I broke her and she broke me back so many times. I think it was really good that I started well in both tiebreaks and that I was leading quite fast. I just tried to focus and fight."

Muguruza came into the tournament without back-to-back wins since the semifinal run at the French Open, but dropped just five games across her opening two matches in a blistering return to form in Wuhan.

Her hot streak continued to begin the third-round encounter, as she denied Siniakova multiple early in the set en route to earning a big lead. She had three chances to break for 2-0, and later on a chance to hold.

Muguruza nonetheless shut down all her chances, attacking every short ball and staying aggressive at the net, while using her height to cover the angles, and broke twice to get ahead 5-1.

Siniakova already had experience with a big comeback in the last round - as she came back from a set down and saved a match point to knock out Garcia - and did so again against Muguruza.

The Czech got both breaks back as Muguruza’s unforced errors started to mount, and later saved a set point in the 12th game as Muguruza tried to serve it out for the third time.

The Spaniard won the first point of the ensuring tiebreak but then it was all Siniakova, who won five points in a row to open up a sizable lead and claim the opening set after a hard-fought 65 minutes.

With momentum against her, Muguruza immediately went down a break, but seemed able to put it behind her, along with the disappointment from the first set.

The two-time Grand Slam champion dialed up the aggression and curbed the errors coming off her racquet, and got the break back to level at 2-2 - before another break extended her lead to 4-2 as Siniakova’s frustration grew.

However, it was the Czech's turn to go on another run, as she pocketed three straight games from there to earn herself a first opportunity to serve for the match.

A set which ultimately saw eight breaks of serve eventually went through to another tiebreak, as Siniakova never reached match point in her first two chances to serve for a 10th career Top 20 victory.

An erratic effort from Muguruza characterized the tiebreak, as the Czech won the first five points to win the match in two hours and eight minutes, and become the seventh unseeded player of the eight in the quarterfinals. 

"I feel really confident on the baseline, and I can run...I just try to put so many balls in. I can be on defense, and then from defense I can play agressive. I feel very confident playing here and I hope it can continue," Siniakova assessed of her week thus far.

"I played in the U.S. and I lost in the first round twice, and then I did well in New York, but I think [qualifying] was really good for me. I played two matches, got confidence and I prepared on this court. [This result] is very big for me...it means a lot, I think I can be here and I was working for this. I'm just trying to improve my game, and it's not the end."