Even though their head-to-heads tend to disappoint, the sibling rivalry between Serena and Venus Williams is very much alive.

They have a personal battle to see who can win the most titles this season and are neck-and-neck after Serena won the Princess Cup in Tokyo on Sunday.

Each have seven tournament wins, with Serena's tally including three Grand Slams; Wimbledon plus the French and US Opens.

On Sunday Serena did not have it all her own way in the final against Belgian Kim Clijsters, and she trailed by a set before getting her game into gear.

After a 2-6 6-3 6-3 win over Clijsters, she admitted: "It was a tough beginning as she played so well, and I was making too many errors.

"At the end of the second set, I was starting to return around the level I was at the US Open, which is great.

"At one point I said to myself, 'If you don't move up or stop making errors you're going to give it to her'.

"But I decided to keep hitting as there is no point being a chicken about it. When I lose I go out hitting."

While Serena at the moment is top of the world rankings and on a hot winning streak, she knows that cannot last - particularly with Venus wanting her number one ranking back.

"Nobody can win all the time. She [Clijsters] played really, really great in the first, second and third sets," added Williams.

"I just lifted my level in the second and third.

"I love playing here because the people are so nice and they cheer for me. Usually people don't cheer for me as they are cheering for the underdog.

"So it's a nice change to play here."

Tournament hosts at the Ariake stadium were forced to close the roof on the venue due to rain in the Japanese capital, and a floodlight failure caused another delay.

It made concentration difficult for both players, but Clijsters was coping best and took two early breaks.

She closed out the set but relinquished serve in game eight of the second set and Williams held to level up the match before steamrollering through the decider to reclaim the title she won two years ago.

Williams is now unbeaten in eight matches at the tournament and has a 5-0 career record against Clijsters.

Aside from the grand slams, Williams has titles this season from Scottsdale, Miami, Roma and now Tokyo.

She will remain at number one in the world rankings, with Clijsters expected to stay eighth.

Venus is at number two and Serena recognises her sister's threat still looms large.

It is nigh-on impossible to envisage any other player on tour becoming world number one over the next five years, and to hear the pair still have a burning hunger for the game is encouraging.

That has been a factor questioned in recent years, with even father Richard Williams suggesting they learn a second trade.

Serena said: "Venus is so great, she never loses outside the Grand Slams.

"If I'd won in Los Angeles, maybe I'd be ahead right now, maybe if I win next week in Leipzig, but she's playing Moscow the week after and I think she'll win that."

The Australian Open next year is the big goal on the horizon for the pair, and it is a tournament Serena missed this year through injury.

Should she win there, and Venus is likely to be the only real threat, the younger of the sisters - Serena turns 21 on Thursday, while Venus is already 22 - will hold all the Grand Slam titles.

No player since Steffi Graf, who won all four in 1988, has achieved that feat.

"My goal right now is the Australian Open, both Venus and I want that one as neither of us have won it," said Serena.

"Maybe I want it more as I want to do the 'Serena Slam', so to speak."