Sloane Stephens struggled to victory in her opening match in Charleston, yet nonetheless scored a much-needed win to reach the last 16.
The top seed needed two hours and 37 minutes to subdue Spanish world No.78 Sara Sorribes Tormo 7-6(4) 7-6(4) at a tournament where she was a champion in 2016.
Meanwhile, in Acapulco, former world No.1 Victoria Azarenka breezed to a 6-4 6-2 win over Japanese qualifier Miyu Kato.
Azarenka next faces Zarina Diyas for a place in the quarterfinals; Diyas accounted for local wildcard Renata Zarazua 7-5 6-4.
Stephens arrived in Charleston having gone a lean 2-3 since she advanced to the fourth round of the Australian Open.
In her title defence in Miami last fortnight, she went down to Tatjana Maria in a listless second-round performance that saw her drop to world No.8 in this week’s rankings.
Stephens trailed Sorribes Tormo 5-2 in the opening set and missed two match points in the 10th game of the second set before finally wrapping up victory.
.@SloaneStephens outlasts Sorribes Tormo, 7-6(4), 7-6(4)!
— WTA (@WTA) April 2, 2019
“I just thought it can’t get any worse and just (tried to) hit through the ball and keep fighting and (accept) there’ll be some ups and downs
“It was a really tough day today and the conditions were tough. I was playing against all the factors and playing against myself a little bit there,” Stephens assessed.
“I just tried to keep fighting, trying to find my way back. She was a tough match up but I’m happy I got through. Try to get some confidence back.”
Stephens, whose clay-court record includes a run to last year’s Roland Garros final, faces a third-round meeting against either in-form 14th seed Ajla Tomljanovic or Slovene Tamara Zidansek.
She was the only second-round winner in Charleston on Tuesday; players winning their first-round matches included 11th seed Danielle Collins, 12th seed Mihaela Buzarnescu and former top-10 player Andrea Petkovic.
Azarenka also fell in this week’s rankings after failing to defend last year’s points from Miami, where she reached the semifinals before going down to Stephens.
Now ranked No.67 – down from 46th, before she lost in the second round in Miami – Azarenka required just 74 minutes to see off Kato.
“I think the first match is never easy, to play against an opponent that you don’t know, and I hadn’t had enough time to adapt to the conditions yet,” Azarenka told wtatennis.com.
“Hopefully it will warm up a little bit.”
Also playing this week in Monterrey are fellow multiple major champions Angelique Kerber and Garbine Muguruza, who are the top two seeds.
Third seed Anastasia Pavlyuchenkova – a four-time champion in Monterrey – opened her 2019 campaign with a 6-0 6-3 win over Anna Blinkova.
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