CINCINNATI, OH, USA - Madison Keys snapped a five-match losing streak against Simona Halep in dramatic fashion on Thursday night at the Western & Southern Open.

In a match that lasted nearly two hours and thrilled a capacity crowd, the No.16 seed beat the No.4 seed, 6-1, 3-6, 7-5 to reach the last eight in Cincinnati for the second straight year. 

It is Keys' first victory in five years over the former World No.1, dating back to their first-ever meeting at the Apia International in Sydney in 2014.

"I think I played really smart tonight. Obviously I played a really good first set, and I don't think she played her best tennis in the first," Keys said after the match.

"I think the third set we both played a pretty high level of tennis. I think it was the first time in a long time that I just played a little bit more within myself and didn't try to do too much too soon.

"She's been No. 1 for a reason, won Grand Slams for a reason. I knew that she wasn't just going to, you know, ever give up or give in. I knew the entire time I had to fully win the match before, you know, I could actually take a deep breath.

"I think I just kind of trusted myself a little bit, and I didn't really hold back on any of my shots. I think I made a couple of bad misses, but at the same time I think I did a lot of really good things."

The American nearly let a big lead slip away in the final set, as she led 3-0 and had four break points to lead 4-0, and also had a match point in the ninth game before closing out the win two games later.

"I think being able to play as well as I did at the end after being out there for two hours, I was really happy with that," Keys assessed.

"I think also the ability to keep bouncing back after being up 3-0 and then having a match point and all that, staying really calm and continuing just to try to get the next point was my favorite part."

The victory for Keys is her first Top 5 victory in exactly a year, since beating another reigning Wimbledon champion at the time - Angelique Kerber - in the same round of last year's tournament.

In addition, it is just the American's second quarterfinal reached since winning the Volvo Car Open in April, and first since the French Open.

An in-form Keys blitzed through the opening set in just over 20 minutes, as she dominated in all facets. The American never faced a break point, lost just eight points in four service games, and won nine of the 11 points against Halep's first serve.

Despite losing serve twice more in the second set, Halep managed to flip the script as her return game grew more effective. She created 10 break points in the set, winning Keys' serve three times, and earned her first love hold of the match to send it to a decider.

"She played unbelievable and was very powerful on court," Halep said. "I knew that she's going fast for all the balls and with power. I knew that this is her game and I expected a very tough start.

"I didn't have the rhythm at the beginning, and then I fought and I got it but not enough to win the match.

"I say that I didn't play my best but I played much better than yesterday and I moved better. It was a good step forward, so I'm going to keep working."

Keys will next face Venus Williams for a spot in the final four, after the American came from a set down to beat Donna Vekic in an earlier quarterfinal.

"Venus is always tough. She's obviously playing some really good tennis to have some really good wins here, and I played her in the past a few times. I think it's gone both ways a couple of times," Keys said.

"She's obviously playing some really good tennis in order to beat Kiki [Bertens] and then Vekic and all that. I'm going to have to see what she's been doing well."