BRISBANE, Australia - Wildcard Samantha Stosur overcame a gulf in ranking, head-to-head and previous tournament history to shock Angelique Kerber 7-6(5), 7-6(4) in an all-unseeded first-round clash between former Grand Slam champions at the Brisbane International.

A first set of extreme momentum shifts was stolen from within Kerber's grasp by Stosur, whose purple patches at its beginning and end bookended a Kerber comeback that had not been quite enough to get the former World No.1 over the line. That up-and-down stanza set the stage for a rivetingly tight second set, eventually squeezed out by the home player in another tiebreak after one hour and 55 minutes of play to seal her first Top 20 win in Australia since defeating Roberta Vinci in the first round of Sydney 2016.

Having stayed loose enough to execute her game to perfection in every important moment, Stosur laughed as she saw the irony - having suffered a number of nervy losses on home soil over the course of her career. "I mean, it's funny," she said. "This is sort of the way I want to be all the time on court. I put more pressure on myself than anyone could ever do, because I really know what I'm capable of and what I feel like I can do out there. So sometimes I probably want it all too bad - and then that's when you kind of get hamstrung and don't perform as you want to. But today I certainly feel like I did stay relaxed and composed. And like I said before, played point by point and did all those things that I want to do all the time, but sometimes doesn't quite come off. I just wanted to put in a good performance and I did that and proved to myself that my good performance can win matches, so that's a good feeling."

World No.98 Stosur, who had won just one match here since 2012 and is yet to progress beyond the second round in nine previous appearances, had also lost six out of nine prior matches with World No.18 Kerber - including the last four in a row going back to May 2015 - and had not defeated the German off clay since the 2011 US Open semifinals. Kerber, by contrast, had never fallen before the Brisbane quarterfinals in five previous appearances, and in 2016 was runner-up to Victoria Azarenka.

Yet it was the Australian who started more confidently, dominating the opening passage of play with a series of powerful forehands and impressive net reflexes. Though Stosur would wobble to concede her opening break with a double fault, Kerber - whose groundstrokes were unusually error-prone to begin with - repaid the favor in the next game, leaving the path clear for Stosur to build a 5-2 lead.

It was at this point that the three-time major winner woke up, though. Redirecting pace with her forehand brilliantly, a pair of rapid-fire holds sent the message that Kerber had raised her level. Serving for the set, Stosur found herself under pressure from her opponent's forehand - and as one of her own found the net, Kerber broke back, the centrepiece of a run of five games from 2-5 to 6-5.

Though the former World No.1 was unable to serve the set out either, this seemed a minor blip as she rebounded to take a commanding 5-1 lead in the tiebreak. But it was Stosur who took advantage of a pair of netted Kerber backhands to roar back, holding her nerve to pound a pair of brave forehand winners away on the final two points of the set.

This stretch of play exemplified what Stosur discussed about her ability to stay relaxed: afterwards, she admitted that she didn't know what she had changed trailing 1-5 - because she had managed to block out the score. "Every one you always want to play just point by point by point - and I guess considering that I kind of forgot that I was down 5-1, it shows that I was doing that," she said with satisfaction.

Overall, though, Stosur had no complaints about her level throughout the set, despite the scoreboard fluctuations. "I don't feel like I necessarily started playing poorly when I was up in the set," she said. "I was still hitting the ball well, but... then I got back to keeping it out of her hitting zone, using the slice to keep the pace out of the rally, and then when I had an opportunity I took it. So I probably just got back to that more than anything."

There would be no such dramatic momentum swings in the second set, which instead provided a gripping spectacle as both players, now playing well simultaneously, went toe-to-toe for 61 minutes. Stosur's forehand and volleying continued to work a treat, and the 35-year-old was even confident enough to mix up her drive and slice backhands to good effect. Kerber was more intermittent, with uncharacteristic errors continuing to beset her from both sides - but the left-hander was able to find her best tennis to stave off Stosur, saving two break points in the third game and another in the seventh in fine fashion.

But Stosur's performance would be characterized today by her refusal to blink, even when opportunities had apparently slipped past. Serving second, the former World No.4 was able to come up with the goods on serve throughout the set, including a key six-deuce tussle in the eighth game in which she had to save a break point of her own.

In a microcosm of the set, the inevitable tiebreak began with both players hitting their spots with a sequence of excellent points - but midway through, it was Kerber who began to miss by inches, enabling Stosur to inch into the lead. And just as in the first set, Stosur was able to take full advantage, landing first serves and forehand blows before sealing victory on her first match point with a service winner to set up a second-round clash against either No.8 seed Madison Keys or qualifier Marie Bouzkova.

Stosur was immediately able to recall her most recent encounters with both: prior to today, defeating Keys 6-4, 4-6, 6-4 in the second round of Miami last March had been her most recent Top 20 scalp; conversely, Bouzkova had dealt her a 4-6, 6-2, 6-2 first-round loss here last year. On the American, Stosur said that the key would be "being smart with where you want to hit [the ball], keep good depth, still play your game but know that you know what's coming from the other end"; while a Bouzkova rematch, she said, would mean that she would "look back to those notes for sure and see what I can change and do better."

2020 Brisbane highlights: Stosur holds nerve to stun Kerber