For the second round in a row, Leylah Fernandez staged a remarkable comeback to knock a former champion out of the US Open -- and the Canadian teenager is now into the first Grand Slam quarterfinal of her career.

In a battle of left-handers, World No.73 Fernandez fought back from a set and a break down to stun No.16 seed and 2016 US Open titlist Angelique Kerber of Germany 4-6, 7-6(5), 6-2 in 2 hours and 15 minutes, one day before her 19th birthday.

Read more: Leylah Fernandez on beating Osaka, self-belief and the power of family

Fernandez had knocked defending champion Naomi Osaka out of the draw with a come-from-behind victory in the third round. Her fourth-round tilt against Kerber was nearly a carbon copy, as the Canadian again gritted her way through a second-set tiebreak before ousting Kerber.

Stat corner: The former World No.1 Kerber came into the match having won 17 of her past 19 matches, as she had pulled off a resurgent summer, which included her first WTA singles title in three years in Bad Homburg and a run to the semifinals at Wimbledon.

But Fernandez, one of two 18-year-olds to have reached the round of 16 this week (also Emma Raducanu), persevered through another comeback win in New York, slamming 45 winners past Kerber in the topsy-turvy affair.

After pulling through the tiebreak to extend the match, Fernandez was more clutch when opportunities came her way in the decider. Both players had three break points in the third set, but Kerber could not convert any while Fernandez went 2-for-3.

Key moments: From 2-4 down in the first set, Kerber ramped up her footspeed and started to steer the direction of the ball into challenging positions, helping her go on a tear. She found a backhand winner down the line to garner a break at love and a 5-4 lead, then slammed another winner off of that wing in the next game to convert her first set point -- her ninth point in a row.

But Fernandez’s forehand started to become a more consistent weapon during the second set, and at 3-4 down, she was able to use that to obtain a love break and get back on serve. The Canadian continued to move forward, holding a set point at 6-5, but Kerber triumphed in a run of lengthy rallies to edge into the tiebreak.

In the breaker, Fernandez found rocketing groundstrokes as she swept to a quick 5-1 lead, and despite letting the next three points slip away, another powerful return winner gave the teen two more set points at 6-4. A Kerber forehand winner saved the first, but on the second, Fernandez forced an error with a deft drop shot, and the match was tied at one set apiece. 

In the third set, Kerber saved a break point at 1-1, but she was less successful at 2-2 as Fernandez roared through the game to clinch the first break of the set. An ace to consolidate for 4-2 propelled Fernandez even further ahead, and after saving two break points, she served out the match at 5-2, winning the last five games of the match in the process.

Words from the winner: "The match was incredible," Fernandez told the press after her win. "I was playing well in the first set. Unfortunately I did a few mistakes. Her, as a great player, great champion, she noticed and she took advantage of it. I was able to refocus and fight back for the second. In the third, I was just enjoying every moment of it.

"At 3-4 [in the second set], I don't know if it was just a mindset or it was a feeling, but I just felt like that was the moment that I was going to break her serve and I was able to figure it out. I was just glad I was having fun on the court. I was trusting my game at 100 percent.

"I have been working hard every day since the day I started playing tennis and since the day I set my mind to being a professional. I expected that one day my tennis game is going to come through and that I'm going to be on the big stage in front of a big crowd playing against big players and also getting the wins."

Next up: In the quarterfinals, Fernandez will take on No.5 seed Elina Svitolina of Ukraine. The 2019 US Open semifinalist Svitolina dispatched No.12 seed Simona Halep earlier on Sunday.

Fernandez and Svitolina have faced off once before, with Svitolina winning their quarterfinal clash in Monterrey last year.

Sabalenka blasts past Mertens

No.2 seed Aryna Sabalenka of Belarus made it into her first US Open quarterfinal with ease on Sunday evening, storming past her good friend, No.15 seed Elise Mertens of Belgium 6-4, 6-1 in 1 hour and 10 minutes.

"I'm just really happy to be in the quarterfinals here because I really like playing at the US Open, especially with the crowd, and the atmosphere is really enjoyable here," Sabalenka said afterward.

Sabalenka and Mertens have teamed up to win two Grand Slam doubles titles together, and each of them have taken a turn at doubles World No.1 during that run (Mertens currently still occupying the top spot in doubles).

But Sabalenka had a big 5-2 lead in their singles head-to-head coming into the clash, and she continued to dominate proceedings as she reached her second straight quarterfinal at a Grand Slam event. Sabalenka made her first major quarterfinal or better with her run to the Wimbledon semifinals earlier this summer.

Sabalenka had 22 winners to Mertens' 13 in the match, and despite slamming 10 double faults, the World No.2 booked her trip into the final eight after converting four of her five break points.

Sabalenka started to roll at 5-4 in the first set, when she broke Mertens at love to seal the one-set lead. In the second set, the Sabalenka backhand in particular started to flow on important points, with that wing helping her break for 3-1, and save a break point after a double fault en route to a hold for 4-1.

All sectors of the power game were working for Sabalenka. She returned with aplomb to earn a double-break and a chance to serve out the match at 5-1, and there, an error-forcing forehand sealed the routine victory for the second seed.