CINCINNATI, OH, USA - World No.1 Simona Halep won the final six games of a two-day clash with Ajla Tomljanovic to advance, 4-6, 6-3, 6-3, in the second round at the Western & Southern Open.

"It was really difficult because we were waiting so much," Halep said after the match. "I'm really happy I could go through this. It was tough, but I'm happy to play again tonight."

Halep was fresh off her third title of 2018 at the Coupe Rogers but struggled early on against the 25-year-old Aussie, a former Top 50 player. Rallying from a set down, Halep stood strong through rain delays that ended play Wednesday night and slowed her start on Thursday to win in two hours and eight minutes.

Tomljanovic was in search of her first Top 20 win since the 2017 Miami Open, when she ended the winning streak of then-BNP Paribas Open champion Elena Vesnina, and enjoyed a bright start to her match with Halep, breaking first and holding off the top seed's fightback to capture the opening set behind a flurry of winners.

Halep began to find her footing even as Tomljanovic took a medical timeout for an upper left leg injury, leveling the match and forcing a decider with more consistent play.

Tomljanovic flipped the script to start the final set with back-to-back service breaks, getting within three games of the upset as Halep was soon plagued by a back injury.

The reigning Roland Garros champion went on to win four straight games and got within six points of victory before rain delay began.

Ultimately, Halep dug into her vast experience upon Thursday's resumption, dropping just two points to claim victory over the Aussie.

In all, Halep struck 21 winners to 38 unforced errors, while Tomljanovic hit 24 winners but 46 unforced.

Halep will return later tonight against No.16 seed Ashleigh Barty in a rematch of their semifinal encounter in Montréal.

"It's a tough opponent. We played in Cananda last week, so I expect a tough one. Hopefully, I will be at my highest level of tennis to win the match."