SAN JOSE, CA, USA - No.7 seed Maria Sakkari had to battle hard to solve the riddle of qualifier Mayo Hibi in a gruelling second-round encounter at the Mubadala Silicon Valley Classic - but ultimately passed the test with flying colors, 4-6, 6-4, 6-2 in two hours and 35 minutes.
Forced to readjust her game plan and commit to a lengthy battle against an opponent whose touch and defence threatened to undo her, Sakkari eventually got hold of her offence to finish with 52 winners to 49 unforced errors, while Hibi ended with 17 winners and just 14 unforced errors.
The pair had met once before, in the first round of an ITF W25 event in Merida in 2014 - but any hopes Sakkari had of reprising her straightforward 6-4, 6-1 win in that match were ground down over the course of a first set that lasted over an hour and drove the Greek to vocal frustration several times.
"I know I didn't play great, but I came back. I never stopped believing that I could win," Sakkari said on-court after the match.
"I fought hard and...that's why I think I turned the match around. I had to be more patient. I was not patient at all in the first set-and-a-half.
"With some good shots and some good serves, I think my serve was a big key for my game today. I'm very happy that I went through this battle today."
Hibi had proven her comfort with arduous marathons under the Californian sun with her maiden WTA main draw win in the first round, a two-hour, 48-minute epic defeat of Timea Babos, and the Japanese player inflicted a death by a thousand cuts on Sakkari in the first half of the match.
Eschewing power in favour of delicately caressing her groundstrokes around the court, Hibi forced Sakkari to generate her own pace on nearly every stroke - before immediately taking it out again with knifing slices and floating forehands.
Hibi's spectacularly hands on defence - no matter how hard Sakkari hit, the ball would seem to come back - meant that last year's runner-up was unable to consistently hit through her, but whenever Sakkari backed off, the extra time permitted the World No.265 to conjure up her own offensive magic in the form of dropshots that died on court and feathery reflexes at net.
Hibi - bidding for her first victory over a Top 100 player since 2013, when she was 17 years old - was also supremely clutch in the opening stages, saving all six break points that she faced in the first set.
Remarkably for a set that saw 15 break points and 17 deuces in total, there would only be one actual break of serve - in the fifth game, one that saw some of Hibi's finest highlights, including a defensive lob immediately followed by a canny net-rush.
Having gone down an immediate break in the second set, Sakkari had to buckle down to force her way back into the match - which the 24-year-old did with superior patience in constructing points, but also some shotmaking brilliance of her own.
The Rabat champion's backhand on the run proved particularly impressive, conjuring angles that even the fleet-footed Hibi was unable to reach.
With a rejuvenated Sakkari seizing three straight games for a 3-1 lead, Hibi made a valiant attempt to save the set, coming up with a pair of fine dropshots en route to breaking back.
But by now, Sakkari had committed herself to a more stoic mentality - as well as to smarter strategy in penetrating the Hibi defence, such as wrongfooting winners and well-timed net forays.
The smash - which had let Sakkari down at key moments in the first set - was now clicking, and she was able to counter Hibi's finesse with some remarkable reflex volleys of her own.
Serving to stay in the second set, it was Hibi who seemed to run out of steam as her game began to leak uncharacteristic mistakes. As Sakkari ratcheted up the intensity to pull away, Hibi's reflexes at net increasingly deserted her - as did the efficiency of her serve.
Though the 23-year-old was able to cling on to break Sakkari twice in the decider - once as the World No.30 served for the match - she conceded her final five service games as the contest fell away from her.
Sakkari's reward for triumphing in this battle will be another test against supreme defence: a meeting with No.1 seed Elina Svitolina in the quarterfinals, after the Ukrainian beat wildcard Daria Kasatkina, 6-3, 6-1.
"I really like the courts here, because they're bouncy and they suit my game well," Sakkari said after the match.
"I love California. I really love it here. If I play the right way and I do the right things...playing here in San Jose feels very, very good, and good for my game."