World No. 5 Jessica Pegula and former French Open champion Jelena Ostapenko prevailed in gruelling three-setters to make the quarter finals of the Adelaide International on Wednesday.

The second-seeded Pegula battled back from a set down to overcome lucky loser Bernarda Pera 4-6, 6-2, 6-3 in an all-American clash ahead of her campaign at the Australian Open, which begins on Sunday.

Jelena Ostapenko during her match against Caroline Garcia on Wednesday. Mark Brake/Getty Images

Pegula looked ill at ease against Pera’s powerful left-handed game in the opening set and converted only one of her 10 break point chances.

But she found her feet in the second set to draw level and after nosing ahead by a break at 4-3 in the third, she then saved five break points for a crucial hold.

Her sixth break in the next game vanquished Pera, setting up a quarter-final against Anastasia Pavlyuchenkova on Thursday.

“(Pera) was hitting unbelievable shots in the first set, I didn’t really think there was much more I could do,” Pegula said.

“I just tried to change a couple of things, and stay relaxed, not get frustrated that she was playing some great tennis. She’s always very tough.”

Ostapenko had an injury scare before overpowering Caroline Garcia 6-4, 5-7, 6-4 to reach the quarter finals.

Leading 5-4 in the opening set of what turned out to be a hitting contest, Ostapenko took a medical timeout after complaining of sharp pain in her left thigh.

She returned to secure the set but Garcia forced the decider with her spirited comeback in the second.

Ostapenko, playing with a strapped thigh, broke her opponent in the opening game of the third set and made her fourth match point count to prevail in two hours and 41 minutes.

“I’m really happy that I won this match,” said the feisty Latvian, who meets Marta Kostyuk for a place in the last four.

“I’m a real fighter. Honestly, it was a little bit tough to play because of the pain, but I somehow won the match.”