Rafael Nadal has refused to offer the specifics of his frosty exchange with Lorenzo Sonego after a straight-sets win over the Italian.

The Spanish star beat the No.27 seed 6-1 6-2 6-4, and seemingly had a lecture for the 27-year-old at the net after match point.

Despite refusing to reveal exactly what he told Sonego, Nadal was adamant his message to the Italian was "in a nice way".

"I wanted to tell him something. I did it in a nice way. I feel really bad now if I bothered him," he said.

"I was wrong. Probably, I should not have called him on the net. So, apologise for that. My mistake on that. No problem to recognise that."

Nadal seemed to be irked when Sonego convinced officials to close the roof so that the floodlights could be turned on, with the 36-year-old Spaniard leading 4-2 in the third set.

After a 10-minute delay, Italian riled him further by grunting in the middle of a rally after hitting a spectacular shot.

Nadal ended up dropping his serve for the first time in the match and, like a stern headmaster, he beckoned Sonego to the net and gave him an earful about the etiquette of tennis.

After the players shook hands, another lengthy discussion took place at the net as a visibly upset Sonego tried to understand exactly what he had done wrong.