CINCINNATI, OH, USA -- Seven-time Grand Slam winner Venus Williams of the United States pulled off a stirring upset in the second round of the Western & Southern Open on Tuesday, knocking out defending champion and No.5 seed Kiki Bertens of the Netherlands, 6-3, 3-6, 7-6(4).

"I'm playing at home and we know [Bertens is] playing great tennis, so that was cool," Williams said, during her post-match press conference. "For me, it's just round by round. I have a third round to play. To be honest, every player I play plays amazing. She was no less than that."

"It was pretty hot," Williams said, regarding the stuffy conditions. "You just have to keep focused on hopefully keeping your feet going. I'm not sure how she felt about the heat, but, you know, I think we were playing at probably the toughest slot during the day. We had to be superheroes out there."

A three-set tussle was expected as the duo had faced off twice last year with both of those matches going the distance. Each player had  picked up one win -- Williams triumphing in Miami, and Bertens leveling the rivalry at Wimbledon -- and it was Williams who claimed Tuesday’s rubber match after two hours and 17 minutes of play.

GALLERY: Snapped in Cincy: All the best photos on and off the court

"If I could figure it out, I would try and win in straight sets," Williams smiled, when asked about their long matches against each other. "I'll have to give a long think to that one."

This was former World No.1 Williams's second Top 5 victory of the season, alongside her win over then-No.3 Petra Kvitova at Indian Wells. Bertens had 11 aces and won 71 percent of points on her first serve, but Williams was more effective on second serve, winning 55 percent of those points, while Bertens only won 42 percent of her second-service points.

"I just tried not to go too big, because I can go so big and I have a lot of power and it's not always easy to control it," Williams stated. "So I'm trying to play smart instead of going hog wild, which is extremely easy to do."

Williams, who now finds herself in the round of 16 in Cincinnati, awaits the winner of Wednesday’s second-round clash between Donna Vekic of Croatia and another former World No.1, Belarusian Victoria Azarenka.

"To me, I just want to win, so I don't know what's gonna happen," said Williams. "Those ladies [Vekic and Azarenka] are both amazing players, and I think it will be an exciting match."

After holding for 2-1 with a deft dropshot, Williams claimed the first break of the match after a clutch of errors by Bertens put the American up 3-1. Williams dropped serve in the next game but quickly regained the advantage, earning a break point with another winning dropshot and converting her chance to lead 4-2.

Williams was fantastic in the forecourt for the remainder of the set, punching volleys for winners to hold for 5-2. Serving for the set at 5-3, Williams grabbed double set point by withstanding a long rally, ending with a wide Bertens backhand error. The former top-ranked player in the world needed only one, after a netted forehand miscue by the Dutchwoman ended the opening frame.

In the second set, Bertens jumped ahead an early break for 2-0. The Dutchwoman plopped a shot over Williams’s head down the line to reach triple break point at 3-0, and garnered the 4-0 lead after a long forehand miscue by Williams.

The American fought back, though, claiming both breaks back after patches of errors by Bertens, and Williams found herself back on serve at 4-3. But Williams misfired on groundstrokes in the following game, as Bertens reclaimed a break advantage, moving ahead 5-3.

Serving for the set, Bertens again suffered through a period of unforced errors, and Williams held two break points to get back on serve once more. However, netted groundstrokes by Williams made those chances disappear, and Bertens eventually closed out the game and the set with two consecutive aces.

Bolstered by some excellent volleying, Williams took the early lead in the deciding set, racing ahead 3-0. The American saved some of her best serving for the final frame, as she eased ahead to a hold for 5-2. However, the American faltered when serving for the match at 5-3, blasting numerous errors to allow Bertens back on serve at 5-4.

The Dutchwoman stayed with Williams from that juncture forward, using her patented serve-and-forehand power combo to hold for 5-5, and blasting an ace to hold at love for 6-6 and set up the decisive tiebreak.

It was Williams, though, who took control of the breaker, taking both of Bertens’s first two service points with forehand winners, en route to 3-0. Outstanding forehand depth by Williams forced a netted error from Bertens to give the American a 6-2 lead and four match points. Bertens saved two but Williams converted the third to eke out a whisker-thin win.

"I think at 5-5, I felt pretty calm," said Williams. "Maybe I rushed a little bit at 5-3. There is such a thing as wanting it too much. You have to earn it. She played a great game at 6-5, awesome game. And then the tiebreaker was a lot more fun for me, I guess, than her. It's just nice to get that lead and have a few chances."