World number three Jessica Pegula was pushed all the way before emerging victorious 7-5 4-6 6-4 against Irina-Camelia Begu in Thursday's Charleston Open third round.

Pegula, the top-ranked American and the tournament's overall top seed, may have thought it was going to be smooth sailing after collecting a break-to-love in the match's opening game, but it was just the beginning of a two-hour-and-32-minute battle.

Begu played terrific tennis against a world-class opponent, winning 47 per cent of the match's total points, but she shot herself in the foot with eight double faults while Pegula had just one.

With the victory, Pegula booked her place in the quarter-final against Spain's Paula Badosa after her stylish 6-1 6-3 result over Russian 19-year-old Diana Shnaider.

Badosa now has a 4-2 record since the beginning of March, but both of those losses came to the in-form Elena Rybakina in eliminations from both the Indian Wells Open and the Miami Open.

Meanwhile, reigning Wimbledon and US Open finalist Ons Jabeur needed just 82 minutes to dispatch Caroline Dolehide 6-3 7-5, and combined with her opening victory against Lesia Tsurenko it is Jabeur's first consecutive wins since the Adelaide International in early January.

Jabeur will play Anna Kalinskaya in her quarter-final after the Russian upset two-time Australian Open champion Victoria Azarenka 6-4 7-6 (7-5).

It was a strong day for the Russians, as Ekaterina Alexandrova got the better of Julia Grabher 6-4 6-2, and world number eight Daria Kasatkina knocked out Bernarda Pera 6-3 7-6 (7-3).

Olympic gold medalist Belinda Bencic won the longest match of the day in a two-hour-and-45-minute war of attrition against Shelby Rogers 4-6 7-5 6-2, but the Americans had something to cheer for in the late window as Madison Keys advanced past Magda Linette 6-2 3-6 6-1.