Fifth seed Sloane Stephens moved into the quarter-finals of the ATX Open as one of only three seeds remaining after a straight-sets win over Heather Watson on Wednesday.

The 29-year-old American triumphed 6-4 6-4 over Watson in one hour and 27 minutes in Austin.

The 2017 US Open champion firmed into favouritism in Austin with third seed Anastasia Potapova knocked out by USA's Katie Volynets in a thrilling three-set encounter.

Volynets came from a set down and also 5-0 down in the third frame, needing two hours and 19 minutes to get past Potapova, winning 5-7 6-2 7-5.

Potapova's exit follows top seed Magda Linette's first-round defeat, while second seed Zhang Shuai withdrew due to illness on Tuesday.

Stephens has seven career WTA titles to her name, the last coming in February last year at the Abierto Zapopan in Mexico.

Varvara Gracheva defeated Anna Blinkova 6-3 6-7 (5-7) 6-1, while American wild card and Texas local Peyton Stearns beat Mirjam Bjorklund 6-3 7-5.

At the Monterrey Open in Mexico, third seed Donna Vekic progressed to the quarter-finals with a 6-3 6-2 victory over Emma Navarro.

Fifth seed Zhu Lin accounted for Rebecca Marino in three sets, while Ysaline Bonaventure also needed three frames to beat Kamilla Rakhimova.