By John Stonestreet

LONDON (Reuters) - A daunting double over the Williams sisters may stand between Angelique Kerber and a second grand slam but it is a challenge the German, having beaten Serena in January to win her first major, feels well prepared to meet.

The evidence suggests that fourth seed Kerber, just as she did when stunning the world number one in the Australian Open final in January, has timed her tournament run perfectly.

Her game has improved with every round and, having struck a finely calibrated balance between accuracy and aggression in Tuesday's pulsating quarter-final win over Romanian fifth seed Simona Halep, is the only woman in the draw yet to drop a set.

"I'm playing really good tennis right now... I think I'm playing like in Australia," she told reporters after her 7-5 7-6(2) win. "I know I can win such tournaments."

Next up for the in-form German is eighth seed Venus Williams, at 36 the oldest player in the women's singles and experiencing something of a renaissance in reaching her first grand slam semi-final since 2010.

When Kerber turned professional, the American already held four of her seven major singles titles and was a star on the horizon for the 15-year-old newcomer.

Since then, her relationship with both sisters has moved onto a more equal footing, and Kerber has beaten Venus three times in the five matches they have played.

"When I came onto the tour the two Williams sisters were always the top favorites but in the meantime I have won against them both and so the respect is there," the German said.

"So my perception of them both has changed over the years."

FIFTY-FIFTY

Kerber is also in a happier place than at the French Open in May where, feeling the weight of expectation after her Melbourne triumph, she lost in the first round to unseeded Dutchwoman Kiki Bertens.

"When I arrived in Paris I was feeling much more pressure... Also, I was not handling it so well off court," she said.

"When I arrived here, I was telling myself, just like in Australia, just be relaxed... just be focusing on the tennis thing."

That confidence is reflected in her willingness to broach the issue of how she might change her game to cope with the different threat of Venus, and the admission that her serve -- the part of her game misfiring on Tuesday -- needs to improve.

"I think Venus makes more mistakes (than Halep) but also more winners, so it will be a completely different match," she said. "I will try to serve better ... but when it comes down to it I will have to go out and take a grip on the match."

Kerber rates her chances of beating the older Williams at 50:50.

If that toss of the coin favors the German and Serena comes through her semi-final against unseeded Russian Elena Vesnina, a repeat of January's major final beckons.

On paper, the younger Williams, 34, has the edge with a 5-2 lead in head-to-heads. But Kerber won their biggest match.

Serena would also be trying to make good on failed attempts at the last three majors -- in France, Australia and last year's U.S. Open -- to equal Steffi Graf's professional era record of 22 major singles titles.

So, unlike Kerber, the American might have more than just 'the tennis thing' on her mind.

(Editing by Ken Ferris)