Roger Federer has backed Serena Williams' criticism of "inconsistent" anti-doping procedures, saying he sometimes has the same frustrations.

A United States Anti-Doping Agency (USADA) database shows Williams has been subjected to five tests so far this year. In comparison, sister Venus, Sloane Stephens and Coco Vandeweghe have all been tested twice and Madison Keys once.

Williams has complained that her issue is with players not being tested equally and Federer fully understands where she is coming from.

"If they rock up 10 times a day, it gets rough - because they can," he said.

"I've had a lot of testing the last month. I think I've had seven tests, so it's been quite heavy. In the village I live in in Switzerland, the tester lives in the same village, so it's very convenient.

"I've been tested quite frequently out of competition. In Dubai, I've hardly ever been tested, which is quite disappointing. In 15 years I've been going there, it's been one test.

"It really varies from place to place. Maybe that's the part I don't like - the inconsistency of where they test.

"What's important is that these people are professional, they know what they're doing, they treat you like humans and not like criminals.

"I understand the frustrations at times. I have them too."

Williams returns to Wimbledon for her first time since 2016 having missed last year's tournament as she was pregnant with her first child.

Federer, who has hailed the American as the greatest of all time, is delighted to see her back.

"It's great, exciting. I'm happy she chose to take that route and not to walk away from the game, which would have been totally fine," he said.

"Why not? After everything that she's done, it would have been the perfect excuse, exit just to say, 'I've had it.'

"But I'm very excited to see her attempt an amazing comeback. This time with a baby, it's a different life. It's a massive challenge for her, but one I'm sure she's up for."