Korea Open: Dayeon Back stuns Jelena Ostapenko in huge shock, Jessica Pegula cruises past Viktoria Hruncakova

World No. 569 Dayeon Back stunned second seed Jelena Ostapenko with a 3-6 6-1 7-6(4) win at the Korea Open.

The South Korean wild card delighted the home crowd with a three-set win over the world No. 13 to book her place in the second round of the tournament.

It looked like being a routine win for the former French Open winner, who twice broke through Back’s serve to move 4-0 ahead and then took the first set.

But what followed was a surprise turnaround, as Back motored into a 3-0 lead in the second set, breaking the Ostapenko serve in the process and following up with a crucial hold.

She swatted away three break points from Ostapenko to take firm control of the set, and broke the Latvian again to move 5-1 up, before drawing level on her second set point.

Ostapenko needed to respond, but failed to hold again at the start of the third set, only to break back immediately to restore parity.

Two more successive breaks followed to leave the decisive set intriguingly poised at 2-2.

Ostapenko might have thought she had got out of jail while leading 6-5, but failed to convert a match point as the Korean held to take the gripping contest into a tie-break.

Back would prevail 7-4 on her first match point and now faces Australian Kimberly Birrell in the last 16.

Top seed Jessica Pegula cruised to the second round of the Korea Open with a 6-2 6-4 win over Viktoria Hruncakova.

The world No. 4 emphatically broke the Slovakian immediately to love, before doing so again in the third game after a third break point.

Hruncakova was clearly struggling on her own serve, and failed to hold again after being broken to 15 and falling 5-0 behind to the relentless American.

A bagel looked to be on the cards until Hruncakova admirably staved off three set points to get off the mark, but this only prolonged the inevitable.

Pegula drew first blood on her fourth set point with a comfortable hold to 15.

It was a case of deja vu as the second set began in the same manner as the first, with the Slovakian broken again.

Hruncakova hit back by breaking Pegula to level up the score at 2-2, and was looking much stronger on her serve.

Pegula managed to swing the momentum back her way with a decisive break on her second break point, moving 5-4 up to give her the chance to serve for the match.

She did so with aplomb with an emphatic hold to love, and will face compatriot Ashlyn Krueger in the next round.