Rafael Nadal insists he has no issue with umpire Carlos Bernardes despite receiving a pair of time violation warnings during his second-round win at Wimbledon.

The world number one came through a tough match with Mikhail Kukushkin 6-4 6-3 6-4, but Nadal's discipline was questioned even before play got under way on Thursday.

Bernardes warned Nadal as he took his time between the warm-up and the first serve and then, between games, when the Spaniard used a change of ends as an opportunity to switch his shirt.

Nadal has a history with the official, having asked for Bernardes not to referee his matches after the 2015 Rio Open, but the 17-time grand slam champion insists there are no lingering tensions.

"There is no problem with the umpire. I am nobody to say 'I don't want him on my court'," he said. "If I believe that somebody's not doing the things fair with me or was not respectful to me, I just can ask if is possible to have another umpire.

"I can't say 'I don't want this umpire'. I say, 'if it is possible, I would prefer another umpire on my court'.

"It is about what happened in Rio de Janeiro a couple of years ago. For me, personally, he was disrespectful. It was not because of time violations - I can understand the time violations.

"It's not about this kind of stuff. It's about the problem that I had with him in the past. It's about that bad moment. That's all.

"There are no problems with him. When I am slow, I accept the warnings. I do my best to be quick. Sometimes I am not good enough to be that quick. I need my time to think about what to do for the next point, that's all."