Defending champion Maria Sharapova had another tough battle on Friday as she booked her place in the semi-finals of the Stuttgart WTA tournament with a 7-5, 4-6, 6-4 quarter-final win against former world number one Ana Ivanovic.
The 26-year-old Sharapova, who is using the claycourt tournament to help prepare for the defence of her French Open title at the end of May, was again taken to three sets as Ivanovic rallied in the second set.
Having needed three hours to beat Lucie Safarova in Thursday's second round, the statuesque Russian has now played more than five hours of tennis inside 24 hours after needing two hours, 16 minutes to beat Ivanovic.
"It's good to face different styles of playing from different opponents," said Sharapova.
"Ana is a bit more aggressive, while Lucie had a bit more variety in her game yesterday.
"I am happy to get through, but I had to push myself.
"It was a tough battle, it was hard to get my body going again after the last game, but it was good to get another three sets under the belt."
After Sharapova took the first set, claycourt specialist Ivanovic rallied in the second with plenty of aggressive shots on her forehand and is on her way back up the rankings, but lacks consistency.
Sharapova broke Ivanovic twice in the third, in a sequence of three breaks, before her aggressive game finally wore down the 25-year-old Serb, who reached the world number one ranking just after winning the 2008 French Open title.
Sharapova, ranked second in the world, will now play third-seed Angelique Kerber in Saturday's semi-final after the German beat Kazakhstan's Yaroslava Shvedova 6-3, 7-6 earlier.
Qualifier Bethanie Mattek-Sands of the USA, ranked 104th in the world, has made it through to the semi-finals on her first appearance in the main draw after she enjoyed a 6-4, 6-2 win over Germany's Sabine Lisicki.
Having failed to qualify on her last attempt here in 2005, the 28-year-old will now faces face Chinese second-seed Li Na for a place in Sunday's final.
Li enjoyed a 6-3, 7-5 win over fifth-seed Petra Kvitova, but the Chinese faced some nervous moments in the second set after her strong start left her 2-0, then 5-2 up, before her Czech opponent levelled at 5-5, but Li clung on.
"It's never over until it's game, set and match," said the 31-year-old who reached the Australian Open final and is bidding to reach her third WTA final of the year after winning January's Shenzhen tournament.
"Sorry if I gave the fans some nervous moments," she added with a smile.
Mattek-Sands is on her way back from injury and having won their last meeting, a three-setter in the quarter-finals at the 2011 Madrid Open, Li said she is relishing her chance to reach the final.
"I saw Bethanie play earlier on and she looks like a good, aggressive player," said Li.
"We'll see how tomorrow's match goes."