Top seed Alexander Zverev was warned for a taking a picture of a contested ball mark at the Madrid Open during more electric line-calling controversy on clay.

The world number two overcame Spain's Alejandro Davidovich Fokina 2-6 7-6 (7-3) 7-6 (7-0) to reach the last 16.

But the success was marred by another row as Zverev was convinced a backhand from his opponent in the 10th game of the second set had gone wide.

The German told umpire Mohamed Lahyani there was a "malfunction in the system" after it was called in by the electronic system.

When Zverev's appeals came to nothing, he brought his phone out of his bag and took a picture of the mark.

The 28-year-old was given a warning for unsportsmanlike conduct, but put the incident behind him to win the second and third sets on a tie-break.

Although other surfaces have used the system, clay events had largely persisted with line judges, or the umpire coming down from his or her chair to examine the mark left by the ball, when calls were contested.

But this season line judges have been removed from all events on the ATP Tour and many on the WTA Tour.

However, the French Open, which begins next month, will also continue with line judges.

This was the latest controversial line-call incident after world number one Aryna Sabalenka was also given a warning for taking a photo during a match in Stuttgart earlier this month, while former Grand Slam winner Victoria Azarenka posted an image of a call from another Madrid match on social media.

Elsewhere in the Spanish capital, defending champion Andrey Rublev lost 6-4 0-6 6-4 to Alexander Bublik - a defeat which will see the Russian drop out of the world's top 15.

In the women's draw, top seed Sabalenka recovered from losing the opening set to beat Elise Mertens 3-6 6-2 6-1.

But third seed Jessica Pegula and sixth seed Jasmine Paolini were both knocked out.

American Pegula was upset 6-3 6-2 by Japan's Moyuka Uchijima, while Italian Paolini suffered a 6-2 6-1 defeat at the hands of Maria Sakkari of Greece.