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Tim Henman battled through to his fourth Wimbledon semi-final in five years on Thursday.

The British number one finished off his rain-curtailed quarter-final encounter against Brazilian Andre Sa 6-3 5-7 6-4 6-3 on a Centre Court bathed in intermittent sunshine.

But Henman, who has struggled for much of this tournament, will need to raise his level several more notches if he is to reach his first Grand Slam final.

He now plays Australian Lleyton Hewitt in the semi-final on Friday and he will have to reduce the unforced errors and lapses of concentration which marred his match with Sa.

Henman had won the first set of their match, 6-3, in the gloomiest of conditions on Wednesday night before the persistent rains halted play. He looked to be quickly into the groove on a much-improved afternoon weather-wise, serving faster and with more penetration.

But then Henman squandered two set points in the 10th game of the second set and his concentration and confidence seemed to evaporate, Sa going on to take the set to level the match at one set each.

The pair swapped service breaks in the opening games of the third set as Henman's erratic game continued. And the Brit was required to save another break point in the next game as he struggled to find the momentum which has evaded him all tournament.

Frustration was beginning to boil inside Henman as he failed to master the Sa backhand which continually sent balls flashing past him, invariably clipping the lines.

Henman, however, raised his level in the 10th game of the third set, a topspin lob, a series of volleys and a forehand passing shot at full stretch breaking the Sa service to give the Brit the set and control of the match.

At last Henman began to play with a little more freedom and resolve, stepping up the pressure in the second game of the fourth set with a series of lobs to break the Sa service.

The Wimbledon crowd breathed a collective sigh of relief as at last there was a measure of daylight between the players. Suddenly, the Henman serve contained more zip and the groundstrokes more precision and for the first time in the match Henman appeared to exert a measure of control.

With the folks on the Hill raucously cheering him on he went on to serve out the fourth set to become the first man to take his place in this year's semi-finals.

Afterwards Henman said: "I don't think I've played my best tennis but I'm winning and I'm still alive. With respect I'm not really interested in semi-finals.

"The first couple of times it was probably a good achievement but that's obviously not what I'm about now. I want to win this thing."

Sa now believes Henman has as good a chance to win Wimbledon as anyone.

He said: "I think he has a good chance. He has less pressure now he is in the semi-final.

"He may have to improve the serve a little but the rest is pretty solid.

"I felt he served well when he needed to."

He added: "I don't have a personal favourite but I think it would be nice for the country if he won."

Meanwhile Venus Williams will face sister Serena in the Women's singles final after both won their respective matches.

Williams stretched her winning run to 20 matches in SW19 with a 6-3 6-2 victory over Belgian Justine Henin in what was a repeat of last year's final.

After dropping serve early on, Williams came back strongly to dominate the match, her power proving too much for the pint-sized Henin.

Serena later joined her with a 6-2 6-1 demolition of Amelie Mauresmo.

In what was a dominant display, Serena swept to victory on Centre Court to set up a repeat of last month's French Open final which Serena won in straight sets.

Venus, who allowed her opponent just two break points, rated her performance the best of the championships so far.

"This was the best match I've played all tournament," she said. "I played at a very good intensity level. I lifted my game.

"She played well but I was maybe more solid than she was. I was able to play the more important points better, the break points."

The champion added: "I was returning serves better. She didn't get many free points and my ground shots, which haven't been that great, were better today."

Meanwhile, Serena said: "It feels great because in the past three Grand Slams I've competed in I've been in the final of all three.

"At the beginning of this year things weren't looking so up for me but I decided to fight and things just turned around.

"I think I've just grown mentally more than anything because for a time I was a little over-confident but now I'm thinking everyone is a fighter and they're going to fight me with as much strength as they have."

But while the sisters looked forward to their third Grand Slam final between them in nine months, their beaten opponents labelled the Williams domination unhealthy.

Mauresmo said: "I think it's a little bit sad for women's tennis. I think people are going to get bored with it - it was already a final at the French Open.

"I can't count how many people since yesterday have told me, 'we don't want a Williams final whatever'."

Henin said: "It's good for them but I think that maybe the crowd like to see other players in different finals of Grand Slams."

In the other men's quarter-finals, Lleyton Hewitt set up a meeting with Tim Henman after scraping through his quarter-final clash with Sjeng Schalken.

Hewitt raced into a 6-2 6-2 lead, but was then forced into a final set by the battling Dutchman.

Hewitt missed four match points at 6-5 in the third set which Schalken took in a tie-break.

The 18th seed then swept through the fourth 6-1 to send the clash into a deciding set.

Hewitt twice went a break down in the fifth but finally came through 7-5 to reach the last four in thrilling fashion and set up that Henman clash.

"You guys have been talking about it and now we have to answer it," Hewitt told the BBC.

"It's going to be a tough match, he is a grass-court specialist and has done extremely well here in the past.

"It's another semi-final for him and I am sure he wants to go one step better.

"But I am young and it's my first time in the semi-finals and so I am just going to go out there and give it everything I have got."

Asked if Hewitt had any weaknesses Henman could exploit, Schalken said: "There are no weaknesses but there are some parts which are very strong.

"His backhand is very strong so I kept away from it the whole match, that was my tactic today. We play the same game from the baseline, that is not the game Tim is going to play.

"That's why he usually has trouble with Lleyton, he likes to come into the net and Lleyton likes to pass. He is a very tough opponent for Tim, he needs to serve very well or he will get in big trouble.

"They can both win and it's going to be an interesting clash. If Tim doesn't serve good he will have some troubles in the match."

Ont he other side of the draw, David Nalbandian became the first South American ever to reach the Wimbledon semi-finals when he beat Nicolas Lapentti.

In what was another long baseline battle, the Argentine won 6-4 6-4 4-6 4-6 6-4 out on Court Two.

He will now play Xavier Malisse in the last four after the Belgian's amazing victory over 1996 champion Richard Krajicek.

Again the match went to five sets and it was Krajicek who cracked first, dropping his serve when serving at 7-7 in the decider.

Malisse held his nerve to serve out and clinch a 6-1 4-6 6-2 3-6 9-7 win.

"It feels unbelievable," said a delighted Malisse. "It's not sunk in yet. I really didn't think I could do it. I was a little tired after the fourth when he was serving better.

"He kept playing great. He gave me a double fault on break point which helped. I'm just really happy. I don't really know what's happening."

Krajicek said: "It was one day of tennis too many in a row.

"I wasn't too happy on the court. The only thing I had going for me was to bluff on his service but he was just serving huge. I just wasn't physically sharp."

Meanwhile, the beaten Lapentti delivered a parting shot at Wimbledon officials after being forced to play on Court Two - an outside court - instead of experiencing the thrill of Centre Court or Court One.

"I think it's stunning that a quarter-final match was played on Court Two," he said.

"I'm going to say some day, 'Well, I made it in the quarters, but I never got to play on Centre Court'.

"I think that was not nice from the officials of the tournament. I played well on Court Two during the tournament, but I would have loved being on Centre Court."