It was Sabalenka’s ninth consecutive victory over Mertens, her doubles partner while winning the 2019 US Open and 2021 Australian Open. Sabalenka had won 14 consecutive sets against the Belgian.
“She’s such a great player, it doesn’t matter the score between us, it’s always a great battle,” Sabalenka said.
Sabalenka will next face either Rebeka Masarova or Peyton Stearns.
Defending champion Rublev was eliminated after a 6-4, 0-6, 6-4 loss to Bublik.
It was the 10th top-10 win for the 75th-ranked Bublik, who made it to the fourth round in Madrid for the fourth time. Rublev did not have to play in the previous round because of an illness to Gael Monfils.
The eighth-ranked Rublev beat Felix Auger-Aliassime in last year’s final in the Spanish capital.
Bublik will next face Ben Shelton or Miami champion Jakub Mensik in the round of 16.
Brandon Nakashima got past Flavio Cobolli to reach the fourth round of a Master 1000 tournament for the fifth time. The 32nd-ranked American will next meet either Daniil Medvedev or Juan Manuel Cerundolo.
