Rafael Nadal believes his quarter-final clash with world number six Tomas Berdych at the Madrid Masters on Friday will be a good indicator as to whether he is finding his best form after two surprising defeats in recent weeks.
The world number one remained on course to retain his title in the Spanish capital with a comfortable 6-1, 6-4 win over Finland's Jarkko Nieminen on Thursday.
Nadal had come into the tournament looking to dispel doubts after suffering shock defeats to David Ferrer and Nicolas Almagro in the quarter-finals of the Monte Carlo Masters and the Barcelona Open respectively.
However, he has looked in fine form in Madrid as he has cruised into the last eight for the loss of just six games in beating Nieminen and Juan Monaco in round two.
Nadal, though, was frustrated not to have closed out the match more comprehensively having led 3-0 early on in the second set.
"For an hour I was probably playing the best tennis that I played for the last three weeks. It was one of the first days that I felt that I was doing with the ball what I really want to do, so that's a very positive thing," he said.
"The negative thing is in the match I had an opportunity to close the match with great feelings and I made few mistakes in a row. That made the match more complicated and I gave Jarkko the opportunity to play more aggressive and with more determination.
"For me it was a mistake to give him that option because I had the match completely under control with 6-1, 3-0. Tomorrow is another is story against one of the best players in the world and I need to be at my 100 percent to have any chance."
Nadal has beaten Berdych in their last 16 meetings stretching back to 2006, but the Czech's last victory over the Spaniard also came in Madrid, albeit on a hard court rather than Nadal's favoured clay.
And Nadal was impressed by Berdych's resolve in coming from a set down to beat 12th seed Grigor Dimitrov 3-6, 6-3, 6-2 on Thursday.
"Berdych played an important match today. To beat Dimitrov, who has been in great form, says a lot for him. I saw the first set and a little bit of the second, but not a lot because I was getting ready.
"He's a very dangerous player on every surface and here even more with the altitude because the ball flies very fast and his serve is very difficult to break.
"My goal is to try to be aggressive. I know that against him I can't leave many spaces on the court. If he strikes the ball in comfortable positions I'm done for because he has great power in his shots."