The Madrid Open has seen plenty of upsets in the first few days of action but business picks up on Friday.
There is the men's quarter-finals with seven players from the top 10 in action, while it is also the women's semi-finals.
Here are some talking points.
Great game today vs @Gael_Monfils,
— Roger Federer (@rogerfederer) May 9, 2019
Thank you fans for the special atmosphere here in Madrid 🙌 pic.twitter.com/hC6kiahtlo
Roger Federer's toughest clay test yet in Dominic Thiem
Madrid is Federer's first clay tournament in three years and he showed his grit on Thursday as he saved two match points before beating Gael Monfils.
Dominic Thiem is a much different proposition. The Austrian won in Barcelona, convincingly beating Rafael Nadal in the semi-finals.
He also has the confidence of beating Federer in the Indian Wells final in March, coming from a set down to triumph.
The 2018 French Open runner-up should give a much better gauge of where Federer is at on clay and it will be a surprise if the 20-time major winner makes it through to the last four.
Rafael Nadal looking better
The Spaniard's form has been patchy at best on clay so far this season, but he has looked better in Madrid.
He hasn't dropped a set yet and he is moving visibly better then in Monte Carlo or Barcelona.
The 11-time French Open champion faces another Roland Garros winner in Stan Wawrinka on Friday.
Wawrinka looks to be finally getting back to where he was before injury wrecked the past couple of seasons, but Nadal should progress here.
Simona Halep's physical challenge
The 2018 French Open winner is clearly labouring with injuries but is still through to the last four.
Belinda Bencic is having a great 2019 and will make a formidable opponent on Friday.
Bencic won the Dubai Duty Free Tennis Championships in February and the Swiss has maintained that confidence and rallied from 5-3 down in the final set to defeat world No 1 Naomi Osaka on Thursday.
Halep's stature on clay is not up for question, but Bencic is the player with momentum and it would not be a surprise if she prevails here.
