Naomi Osaka's unbeaten run reached 13 matches on Wednesday at the Gippsland Trophy thanks to a 3-6, 6-3, 6-1 win over Great Britain's Katie Boulter. 

Ahead of Wednesday's clash at Melbourne Park, Osaka had more familiarity with the 24-year-old World No.371 from Leicester than might've been expected, having beaten Boulter twice on grass in 2018 - at both Birmingham and Wimbledon - in the early stages of their WTA careers.

Since then, Osaka's ascended to the top of the WTA rankings and won three Grand Slam titles, while Boulter is in the midst of rebuilding following an injury hiatus that began as she debuted in the Top 100 just about 24 months ago.

Building on the momentum that saw her come from 6-3, 5-4 down to beat American Coco Gauff on Tuesday, the former World No.82 looked primed to continued her breakthrough week with an inspired performance that earned her an early lead - but Osaka had other ideas, and stayed on course for a quarterfinal berth at the WTA 500 event.

"After I lost the first set... I wanted to fight as hard as I can. She was playing really amazing, so I thought that if I lost at the end of the day, it's okay, but I just wanted to give my best effort," Osaka said on-court after the match.

"I like playing long matches because I feel like I can gauge the condition of the court and that my fitness gets better. I played a short match yesterday, so maybe subconsciously, I felt like I needed to play a long match today."

Later in her post-match press conference, she added: "I couldn't have asked for a better, preparation match. I think it was very different from my first-round match. I can't practice a match like this. So I'm very happy with how it went actually. Even though it was long, I think there are a lot of things that I learned in this match."

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2021 Gippsland Trophy Highlights: Osaka rallies to best Boulter

Before she fully wrestled command of the eventual one hour, 47-minute win, Osaka needed to quell a comeback effort from Boulter one final time. Trailing 4-0 in the second set, the Brit found her spark once more, ran off three straight games and earned a break point opportunity which would've seen her level at 4-4 if converted. 

With Boulter unable to connect on a second serve return at that crucial stage, Osaka earned second life - and lost just one more game through the end of the match. 

"I felt the entire first set and sort of halfway into the second set, I felt like I was on my back foot a lot. I felt like I was letting her dictate most of the balls. For me, I felt like I never really settled that well," Osaka assessed of the turnaround.

"I think in the second and third set I was sort of able to find what I needed to do, sort of impose my pace onto the rally, as well. 

"I was actually more calm after losing the first set because I felt like for me, I just wanted to be able to play well. The biggest concern for me in the first set was winning or losing. But after not playing that well in the first, I just wanted to play well and hopefully push her to a third. So that was my biggest goal in the second. I was able to do that. I just tried to keep it rolling."

Osaka's next opponent will be Romania's Irina-Camelia Begu, who won a titanic third rounder against No.5 seed Johanna Konta in three hours and 23 minutes.

The longest match of the WTA season to date, bettering Sara Sorribes Tormo's win against Bernard Pera from Abu Dhabi by nine minutes, the Romanian needed a staggering nine match points - and saved two herself in the second set tiebreak - to secure a 4-6, 7-6(10), 7-6(4) victory. 

Svitolina rides out "rollercoaster" to beat Ostapenko

Also rallying from a set down to reach the elite eight was No.3 seed Elina Svitolina, who overcame former French Open champion and No.13 seed Jelena Ostapenko, 6-7(4), 6-3, 6-2, in a match that the Ukrainian called "a bit of a rollercoaster."

Having never before played a three-setter in three previous meetings, Svitolina secured victory after an opening set where she trailed, led, and trailed again. From 5-2 down, Svitolina saved three set points and won four straight games to earn the opportunity to serve for the lead, but lost that game to 15 and surrendered a 3-0 lead in the ensuing tiebreak. 

"It was a bit windy and it was really tough to find the rhythm, and it's not easy to play against her because she's going for a lot of shots and you have to really be on your toes," Svitolina added after the two hour, 12-minute win.

"I was a bit flat in the first set and I wasn't happy with the way I started. Afterwards, I was just trying to make an extra ball back and tried to make a difference on the balls I had a chance on."

Despite the deficit on the scoreboard, Svitolina ultimately out-steadied Ostapenko to advance, sprinting out to 3-0 leads in the second and third sets and never trailed from then on. 

"I was very happy the way that I bounced back in the second and third sets. Being more aggressive and putting more balls in the court was key, because I was making a lot of mistakes in the first set."

- Elina Svitolina on the keys to a comeback

Up next, Svitolina will take on No.7 seed Elise Mertens, who edged No.12 seed Caroline Garcia, 7-6(1), 6-3, in performance that the Belgian proclaimed she was "happy" with.

"I played a better match than yesterday [a 6-2, 6-2 win over Japan's Mayo Hibi]. I think everybody was holding serves. There weren't a lot of rallies, but overall I think it was a good match," Mertens said.

"Second set, I served better. I got some more second serves so I could attack. I knew I had to attack the second serve because she's got a really good kick."

Svitolina and Mertens have played four times previously, with the Ukrainian holding a 3-1 head-to-head edge. All four matches have been decided in straight sets. 

"I know her game and it's going to be very different [than today]. It'll be important for me to bring my best game from the beginning," the No.3 seed said. "She tries to be aggressive, so we'll go through with my coach the plan for the match."