Clijsters surprised with her improbable charge

Clijsters's improbable charge into the US Open semi-finals has surprised the Belgian as much as anyone.

Clijsters, who returned to the tour several weeks ago after two years on the sidelines, breezed by China's Li Na 6-2, 6-4 on Tuesday on Arthur Ashe Stadium.

"I never had that kind of mentality before the tournament knowing that I was going to get to a semi-final or final at the grand slams, obviously," said the 2005 Open champion.

But the former world number one is now in the last four, where she will face three-times champion Serena Williams.

Clijsters, who left the game to start a family, said her confidence grew during a round of 16 appearance in Toronto last month where she lost to world number five Jelena Jankovic.

"Something switched with me after Toronto where I felt like, 'Okay, I can compete with these girls.' That was obviously a big question in my mind," Clijsters told reporters.

"Although I was hitting the balls really good in practice and I felt physically I was doing well, you still want to know what it's like to play against those girls and get a feel for how the level has changed, how girls are playing different tennis maybe these days.

"So I am surprised that I'm sitting here talking to you right now, but I'm very happy, and, you know, flattered that I get to do that."

The 26-year-old Clijsters dominated her match against the 19th-ranked Na and avenge a loss she suffered the last time they met in 2007.

Na, who had 41 unforced errors, won three successive games in the second set to take a 4-3 lead but Clijsters responded with a three-game streak of her own to close out the match.

Now Williams meets Clijsters in the last four. The 11-times grand slam champion has won seven of eight career matches over Clijsters.

"She's always been able to step it up at the grand slams," the Belgian said of Williams. "I've seen her play here, and she has that face where she's like, 'I'm here to do business.'

"But that doesn't mean that you don't get chances when you get to play her. The one who keeps the unforced errors down I think is going to get through."

Rafael Nadal survived an early scare against Gael Monfils before marching to a 6-7, 6-3, 6-1, 6-3 victory in the fourth round on Tuesday.

Monfils played some extraordinary tennis to take the opening set but Nadal looked back to his best as he left the French 13th seed in his slipstream on a floodlit Arthur Ashe Court.

Nadal suffered a blip when he was broken in the fourth set to allow Monfils to level at 3-3 but he struck back to claim victory in two hours 45 minutes.

Victory combined with Andy Murray's defeat earlier in the day means Nadal, who faces Fernando Gonzalez in the last eight, returns to number two in the rankings.

Court invader kisses Nadal

Meanwhile, Nadal was kissed by a fan who invaded the Arthur Ashe Stadium court after he beat Gael Monfils to reach the U.S. Open quarter-finals on Tuesday.

The Spaniard was returning to his chair having removed his shirt when the male fan put his arms around the former world number one and planted a kiss on his cheek.

Security staff bundled the fan away quickly while a smiling Nadal appeared unruffled by the incident.

A little nudge in the right direction from his hero and Croatian countryman Goran Ivanisevic helped Marin Cilic to overcome a major hump on Tuesday.

The 20-year-old Cilic, yet another Croatian beanpole blessed with a thunderbolt serve, has long been on the verge of making his grand slam breakthrough.

But after tripping up in the last 16 four times, he made it count on his fifth attempt by clattering past British second seed Andy Murray 7-5, 6-2, 6-2.

"I'm feeling tremendously happy. Of course it's biggest result for me so far. After reaching the last 16 four times, it was a little bit of relief after I won it. Now that I don't have this (mental) blockade in my head, I can look forward."

Cilic began his journey towards grand slam success more than six years ago when Ivanisevic, fresh from his triumph at Wimbledon, needed a hitting partner.

A 14-year-old Cilic jumped at the chance of testing out his skills against the country's best and has never looked back.

"In 2002/2003, Goran was staying in Zagreb and practising all the time with me when he was there. He connected me with his ex-coach, Bob Brett, who I am here with," said Cilic, who watched Ivanisevic's 2001 Wimbledon victory on TV.

"Goran was a great help, otherwise I wouldn't be here today. He is the one who brought tennis on a high level in Croatia and everybody looked at him as a hero.

"For us younger ones, it was also big help money wise as there were many more sponsors coming into the tennis. So Goran helped me a lot."

On Monday, Murray saw just how dangerous the player dubbed "Baby Goran" could be.

With his long limbs and powerful double-handed backhands, he blew away Murray from 5-5 in the first set, conceding just four more games during the rest of the two-hour eight minute contest.

"First set was up and down from both of us. He had a breakpoint at 3-4, and I had break point and he served an ace," explained Cilic, who will next run into Argentine sixth seed Juan Martin del Potro.

"We were both of us searching a little bit for the game to set up everything. When I hold my serve for 5-5, I relaxed a little more and had little more confidence after that.

"He was a little bit struggling to get on my serve to win a few points in a row, so that was key thing for me."

When Cilic faces Del Potro across the net on the quarter-finals, fans may feel as if they are seeing double.

On their bare feet, both stand tall at 1.98 metres (6-foot-6) and both will turn 21 within five days of each other later this month.

However, there is one psychological advantage the Argentine will have over Cilic -- this will be the fourth time he has contested a grand slam quarter-final and in June came within a whisker of defeating Roger Federer in the French Open semis.

But Cilic remained undaunted.

"I played him in (the fourth round in) Australia and I think I have my chance.

"If I take positive things from today... I think I have the game that can hurt him."

Champion Serena Williams won a war of heavy groundstroke rallies to beat battling Italian Flavia Pennetta 6-4, 6-3 on Tuesday and set up a semi-finals clash against former winner Kim Clijsters.

Second-seeded Williams overpowered the 10th seed to break serve at love in the 10th game for the first set, and then earned the only other break of the hard-fought match in the sixth game of the second set to clinch victory.

Williams was dominant in her service games, sending seven aces past Flavia and knocking in 86 percent of her first deliveries.

The Italian, who had saved six match points before advancing past Vera Zvonareva in the previous round, served up five double faults.

"She never gives up, is such a great player and never gave up tonight," the American, who has yet to lose a set in the tournament, said after the 75-minute victory.

Chile's Fernando Gonzalez battered Jo-Wilfried Tsonga into submission to reach the quarter-final for the second time on Tuesday, winning a high octane slug-fest 3-6, 6-3, 7-6, 6-4.

The Louis Armstrong crowd were treated to a furious contest between two of the sport's most entertaining characters, with clean winners streaking off the strings of both players.

Gonzalez, the 11th seed, was giving away five years and four seedings positions to his French opponent but it was the 29-year-old who proved the more durable as his ferocious forehand punched holes in Frenchman Tsonga's defences.

The Santiago master-blaster, a semi-finalist at the French Open this year, will face either Spain's Rafael Nadal in his first quarter-final at Flushing Meadows since 2002.

Juan Martin del Potro has made a fast climb through the ranks of men's tennis and the tall Argentine dreams of making his next step up the grand slam ladder by landing a berth in the final.

"I have the confidence," 20-year-old del Potro told reporters on Tuesday after powering his way past 2003 French Open champion Juan Carlos Ferrero of Spain 6-3, 6-3, 6-3 to reach the quarter-finals.

"I have everything to do a good tournament. I am doing well but I would like to be in the semis or in my first final."

This year del Potro has won on hard court at Auckland and in Washington, beating Andy Roddick in the final. The next week he was runner-up to Briton Andy Murray at Montreal.

The precocious del Potro, who reached his first grand slam semi-final at this year's French Open before losing to eventual winner Roger Federer, has been working on utilising his 6-foot-6 (1.98 m) height to advantage from the service line.

Practice paid off in the fourth round when the sixth-seeded del Potro propelled 22 aces past the 29-year-old Spaniard.

"I improve a little bit my serve," said the Argentine, who hit 137 mph (220 kph) with his fastest serve. "I need to be good with that weapon.

"Today my serve works excellent."

Del Potro reached his first grand slam quarter-final at Flushing Meadows last year, losing to Murray. He also made the last eight at the Australian Open before making his long run at Roland Garros.

He has made a habit of getting quickly to his targets.

Del Potro, at number six, is the youngest player in the top 20. He was the youngest in the year-end top 10 in 2008, youngest in the top 50 in 2007, youngest in the top 100 in 2006, and youngest in the top 200 in 2005.

"I have everything to improve my game and improve my results, so I will try to do it," he said.

"I was so close in French Open to (reach the) final and to beat Roger, but I did semis.

"Here I have another chance to be better, and I will try to go to the semis."

Del Potro had been drawn for another possible clash with Murray in the quarters, but that was derailed when Marin Cilic of Croatia beat the Briton in the fourth round.

"I just think about quarters and Cilic now," he said. "But for sure my dream is to win a grand slam, and this grand slam especially."