ADELAIDE :World number three Elena Rybakina overcame Cristina Bucsa in two hard-fought sets to make the quarter-finals of the Adelaide International on Wednesday, while Jessica Pegula and Jelena Ostapenko had to work even harder to make the last eight.Top seeded Rybakina prevailed 6-3 7-5 against Spaniard Bucsa though the scoreline does not quite reveal how intensely they fought producing 90 minutes of high-quality tennis.Rybakina broke Bucsa in the eighth game, which was just enough for the Kazakh to secure a tight opening set in the Australian Open warm-up event.Bucsa fought tooth and nail but her failure to convert any of the four break points that came her way cost her the set.Rybakina began the second set by breaking her opponent but it was not to be a cakewalk for the top seed.Bucsa finally converted a break point en route to a 4-3 lead but Rybakina somehow managed to dominate the big points and sealed her victory with an ace."I think she played really well today," Rybakina said of her opponent."I didn't start that quick with the movement and everything but I think she played really well, especially coming forward, which you don't see much in women's tennis."It was a bit surprising for me, but yeah, happy that I managed to win."World number five Pegula battled back from a set down to overcome lucky loser Bernarda Pera 4-6 6-2 6-3 in an all-American clash.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