Two-time Mutua Madrid Open champion Simona Halep booked a spot in the third round at the Mutua Madrid Open on Sunday after holding off a late-match surge from an unseeded opponent for the second straight match.

The No.3 seed won the first nine games against China's Zheng Saisai — and served for the match at 6-0, 5-3 — before ultimately wrapping up a 6-0, 6-4 victory in one hour and 19 minutes. 

In her opening victory against Spain's Sara Sorribes Tormo on Friday, Halep also led 6-0, 5-2, but needed to escape late in the second set in an eventual 6-0, 7-5 win.

"It was almost the same [as the first round]... I was really focused in the last game and I just wanted to finish it," Halep said in her on-court interview.

"I think she played really well in the second set. It's a great victory and I'm really happy about it." 

A pitch-perfect performance in 37 minutes allowed Halep to hand out a 6-0 opener for the second match in a row, though Zheng earned herself unconverted opportunities to get on the scoreboard.

The World No.57 had four break points in the set, including three in the first game of the match, and also had two game points on serve in the set's final game before surrendering the bagel. 

2-time champion Halep slides past Zheng: Madrid Highlights

Zheng continued to compete well in the second set: she held from 15-30 down at 0-6, 0-3 to win her first game of the match and broke Halep in the next game, and later, saved a match point while serving at 0-6, 2-5 and again broke serve as the No.3 seed served for victory. 

"I knew from the previous match that I had to just stay there and to stop missing, because she changed the rhythm. She changed the tactics," Halep said. 

"I lost a little bit the timing on the ball. Those high balls were really tough to get, to do something with them. Then I started to miss with the forehand a little bit too much. I just said I have to calm down and just to play... an extra shot inside the court.

"I knew that she's changing the rhythm. She can play very good slice. Also the high ball. So I knew how she's playing, and I actually expected like it was today, and I'm happy that I could stay together there."

Twelve of Halep's 19 winners for the match came in the opening set, and she converted six of the 11 break points she created overall.

Halep is through to the third round in Madrid for the sixth time in 10 career main-draw appearances in the Spanish capital, and will next face either No.13 seed Elise Mertens, who edged Elena Rybakina, 7-6(4), 7-5. The Belgian rallied from a 3-1 deficit in the first set, and saved four set points in the second as Rybakina served for a decider at 5-4. 

Halep owns a 4-1 head-to-head lead against Mertens, including a 3-0 edge on clay. She's never lost a set in those clay-court matches, and has surrendered just 13 games in six sets. The pair's clay-court history also includes a 6-0, 6-3 win for Halep in the second round in Madrid in 2018.

"Mertens, I've played a few times and I kind of know what she's playing," Halep said. "I know that it's difficult because she's going inside the court, stays at the baseline and likes to hit all the balls. It's going to be tough.

"But I'm confident. It will be another match another day, so I will have my chance. I will fight for it."

Elsewhere in Halep's quarter of the draw, two-time Madrid finalist and No.12 seed Victoria Azarenka was forced to withdraw from her second round match against Jessica Pegula with a low back injury.

The American advances to face Azarenka's compatriot No.5 seed Aryna Sabalenka, who advanced over Daria Kasatkina earlier in the day

"I'm very disappointed to withdraw from the tournament. It’s been a really disappointing season for me physically, where at times I’ve felt like I’m playing really well but I’m not able to sustain my level," Azarenka said in a statement. 

"I'm really looking forward to finding the solution and I hope I'll do that soon and be back playing as soon as possible."