Naomi Osaka was offered a dose of reality after her winning start against Serena Williams was ended, but the Japanese claimed defeat meant she "accomplished [her] dream".

Two-time grand slam champion Osaka will return to the world number one ranking next week, yet she crashed out of the Rogers Cup on Friday in a 6-3 6-4 defeat to Williams.

The all-time WTA great is an icon for Osaka; however, the 21-year-old had won their only two prior meetings - including an incredible US Open final in which Williams repeatedly ranted at the officials.

Humbled this week in Toronto, Osaka acknowledged this loss belatedly provided the experience she had always anticipated when facing Williams.

"For me, it's always fun to play her," she told a news conference. "This is someone that I looked up to forever.

"And, actually, this is how I thought the first time I played her in Miami was going to go.

"So in a weird way, losing today, I accomplished my dream. I know that sounds kind of weird, but if there's anyone in the world that I would want to lose to - of course I would never want to lose - I don't mind losing to her, because I learned a lot.

"And this is someone that I kind of modelled my game after when I was little. So this is such a big learning experience for me."

Osaka said Williams played "amazing, which is what she does", and the victor agreed her level had improved from the pair of 2018 defeats.

"I knew [Osaka's] game a little bit more, so that's a little easier," Williams said. "And I'm just overall a little bit better. You know, last year was a miraculous year for me, to be honest.

"So I know her game. I watch her a lot. I knew what I needed to do to hopefully try to get a win today, and it helped a little bit."

As Williams advances to face Marie Bouzkova, Osaka will work to defend her restored ranking more successfully than last time.

Osaka's form fell off a cliff after taking number one with the Australian Open title and she later conceded the pressure had been tough to handle.

She explained on Friday: "I guess I'm excited. But also, at the same time, I feel like that's a position that I think requires a lot of work and a lot of just hardheadedness.

"I can't really let what other people say affect me, and I just have to keep working hard and putting effort. I think that the results, of course, will show, but I have to not think too much about winning or losing."