Nick Bollettieri has urged his protege Tommy Haas to go for broke against Roger Federer in Friday's Wimbledon semi-final.

Veteran Italian coach Bollettieri has known Haas for almost 20 years, since the German moved to Florida to train at his academy.

And he has been thrilled to see the 31-year-old upset the odds by racing through the draw at the All England Club.

But Bollettieri believes Haas cannot afford to rein anything in during his clash with the five-time champion, and stressed serving the occasional double fault should not be a concern, because it will mean he is going for the thumping serves.

"I don't believe that Tommy can compete against Federer from the baseline," Bollettieri told Radio Wimbledon.

"He will probably get a little bit impatient. You've got to also remember Federer covers the court well.

"The key to that match is surprise attack, coming in, throwing in a few double faults on the second serve.

"The second serve has been winning a lot of matches with Tommy.

"When you serve well, as I remember when I was with Boris Becker, you've got to throw in a couple of double faults on the second serve because you're going for the big one."

Haas, the number 24 seed, claimed a shock win over fourth-seeded Novak Djokovic in the quarter-finals, and has been unbeatable on grass this season, following up his Halle title last month with this fortnight's run.

"Tommy's my boy," said Bollettieri.

"He's been with me since he was 12.

"Tommy's a terrific guy and I think the addition of Tommy to the final four is going to make Wimbledon just ever so special."

Bollettieri, who spent the first week of Wimbledon in London, has returned to his home in the United States but has kept a close eye on the tournament and has also been impressed by Andy Murray's run to the last four.

"The other night when I was watching that match [Murray's five-set match against Stanislas Wawrinka], I was yelling at Andy over the television," Bollettieri said.

"His mother was like a Mexican jumping bean - up, down, up, down.

"That was fantastic, and compliments to Andy when he played (Juan Carlos) Ferrero because he had no let down.

"Quite often when you had a match like he did previous to that you tend to let down a little bit.

"Ferrero used to be one heck of a player, hasn't been doing well lately, and that's exactly the time you can get knocked down.

"Andy came, was all business, and here you go, he's at the semi-finals."