By Pritha Sarkar

LONDON (Reuters) - Roger Federer has hooked up with Pete Sampras's former coach Paul Annacone for a "test period" in a bid to rediscover his winning touch.

It was only three months ago that the Swiss, winner of a record 16 grand slam crowns, was world number one and the defending champion at three of the four majors.

"I've been looking to add someone to my team and I've decided to spend some days with Paul Annacone," the world number three said on his website (www.rogerfederer.com).

"As Paul winds down his responsibilities working for (Britain's) Lawn Tennis Association, we will explore our relationship through this test period. Paul will work alongside my existing team and I am excited to learn from his experiences."

Widely considered as the greatest player to have wielded a tennis racket, Federer has shunned full-time help ever since splitting from Swede Peter Lundgren six months after winning his first grand slam at Wimbledon in 2003.

Darren Cahill, Jose Higueras and Tony Roche have made cameo appearances in Federer's players' box since while working with him on a temporary basis.

Just what Annacone can teach a player who has all the shots in the book, and a few more, remains to be seen.

He had reached a jaw-dropping 23 consecutive grand slam semi-finals before losing to Robin Soderling at Roland Garros and Federer has dropped out of the world's top two for the first time in seven years.

The 28-year-old is due to return to action in Toronto in August after taking a month-long break from the ATP Tour.

(Editing by Ed Osmond)