Former World No.1 Karolina Pliskova had to work very hard to finally notch her first match-win of the season and reach the Round of 16 at at the Credit One Charleston Open.

No.3 seed Pliskova of the Czech Republic needed 2 hours and 48 minutes, spanning two days, before fending off the challenge from Ukraine's Katarina Zavatska by the score of 5-7, 7-5, 6-4.

Pliskova missed the first two months of the season after injuring her right hand in practice before the Australian swing. Upon her return in March, World No.7 Pliskova fell in her openers at Indian Wells and Miami.

But Pliskova at last picked up her first win since last season's year-ending Akron WTA Finals with her comeback win over Zavatska.

Zavatska nearly extended Pliskova's winless start to the year. World No.215 Zavatska, who entered Charleston on a protected ranking, grabbed the first set on Tuesday before rain forced a day-long interruption at 3-2 in the second set.

Zavatska, seeking her first Top 50 win, started Wednesday strong as well, moving ahead by a break in the second set and serving for the match at 5-4. Pliskova was two points away from defeat in the latter stages of the set, but she was able to grit out the last three games and send the match into a decider.

Four straight breaks in the middle of the third set did little to clear up the outcome, but at 5-4, Pliskova took her chance, forcing errors from the Zavatska backhand to earn one last service break and clinch the long-awaited victory.

Americans advance early on Wednesday

No.9 seed Madison Keys had a much easier time on Wednesday, as the 2019 Credit One Charleston Open champion eased past Norwegian qualifier Ulrikke Eikeri 6-3, 6-1.

Despite having to deal with an hour-long rain delay in the middle of their match, Keys of the United States needed just 68 minutes of action to end the run of World No.283 Eikeri and make the Charleston Round of 16 for the fifth time in her career.

Charleston: Former champ Keys eases past Eikeri into R3

"I was just happy I was first on and was able to get my match finished," said Keys. "Started a little bit nervous, but I think I played into the match pretty well and felt pretty comfortable out there. Just happy I had a kind of straightforward win today."

No.6 seed Jessica Pegula joined Keys in the Round of 16 with a swift win of her own. Pegula dismissed Italy's Jasmine Paolini 6-2, 6-1 in a whisker over an hour.

Read more: Pegula's clay challenge and her new role in the WTA

Pegula will face fellow American CoCo Vandeweghe in the Round of 16. Former Top 10 player Vandeweghe, a lucky loser out of the qualifying this week, notched another main-draw win with a 3-6, 6-3, 6-3 victory over compatriot Lauren Davis.

A fourth American claimed an early win on Wednesday as No.15 seed Amanda Anisimova dispatched Yulia Putintseva 6-1, 6-2 in an hour and 9 minutes.

But No.12 seed Alizé Cornet of France prevented an American sweep in the early matches by defeating Hailey Baptiste 6-3, 6-2 in an hour and 8 minutes.