At the Qatar Open on Tuesday, Ons Jabeur improved on her promising early-season performance by defeating No. 7 seed Zheng Qinwen to record her greatest victory in several months.
In Abu Dhabi last week, the former world No. 2 defeated Jeļena Ostapenko and forced Elena Rybakina to a final set breaker, indicating that she is returning to her peak during the Middle East swing.
The Tunisian looked in fine form once more as she defeated Zheng 6-4, 6-2 in Doha, her first win over a player ranked in the top 10 since last April.
Wins in 11 of the last 14 games showed that Jabeur was up to the challenge. She later acknowledged that she was feeling confident as she took in the applause from the audience.
"I had a lot of faith in myself when I was training for this match," Jabeur, who will play 2020 Australian Open winner Sofia Kenin in the round of 16 on Wednesday, said.
"That was really beneficial. I made an effort to maintain my attention. She's a champion and really difficult to play against. I'm thrilled to have won, and I aim to continue doing even better in the future.
Gauff lost to Kostyuk in straight sets, and Kostyuk advanced to the third round, where she will face the winner of Frech vs. Linette.
Gauff, rusty from lack of play, made 39 unforced errors and 7 double faults in a subpar performance.
The American got off to a poor start, losing her serve twice and falling behind 4-1. In the ninth game, Kostyuk won the opening set in under 30 minutes, breaking Gauff once again.
In the second set, when she began to settle into her flow, the world No. 3 responded with a break and took a 4-2 lead.
Kostyuk, however, fought back, exploiting a shaky performance from Gauff that ended when she was broken for the sixth time after serving two double faults.
After losing to eventual champion Belinda Bencic in last week's Abu Dhabi semi-finals, Rybakina defeated Payton Stearns in her first encounter.
The fifth seed won against Mirra Andreeva and Rebecca Sramkova to set up a third-round match.
After Rybakina wasted three match points to cut the lead to 6-0 in the second set, she momentarily faltered as Stearns pulled the setback on serve.
On her sixth match point, the former Wimbledon champion finally crossed the finish line.
The French Open and Wimbledon runner-up from the previous year, Jasmine Paolini, also got off to a strong start in the second round of her campaign, defeating Caroline Garcia 6-3, 6-4.
Young Czech Linda Noskova defeated Kazakhstan's Yulia Putintseva 6-2, 6-3 to secure a last-16 matchup with three-time reigning champion Iga Swiatek.