The clown clothes horse of women's tennis is turning into a serious thoroughbred. And that could spell double trouble for Serena Williams later today when she faces Bethanie Mattek on Court Two, Wimbledon's notorious Graveyard of Champions.

Until last month Mattek's only contribution to her sport was a wardrobe of outrageous fashion howlers like the knee-high football socks bought from Harrods and the tiny running shorts and the chandelier earrings the eccentric American wore on Centre Court in 2006.

The previous year Mattek had received a fine for sporting a cowboy hat in the US Open. Then last summer, playing a doubles match in the same Grand Slam, she took to the court in a leopard skin outfit that unfortunately ran out of spots around the area of her cleavage.

In terms of fashion statements, Mattek's attire in recent summers made the white trenchcoat with gold embroidery worn by Williams at Wimbledon last week seem like the epitome of refinement.

Significantly, however, while Williams has so far garnered more headlines for her garb than a two-handed backhand, Mattek has courted little attention in outfits that posed no threat to the All England Club's strict dress code.

It is no coincidence that the uncharacteristically demure 23-year-old has now progressed beyond the second round of a Grand Slam for the first time.

At the French Open she served notice that her priority had become the baseline rather than the catwalk by taking Maria Sharapova to three sets. And she earned the right to face Williams for a place in the Wimbledon quarter-finals by beating 11th seed Marion Bartoli of France, last summer's surprise finalist, at the cost of just five games.

What has brought about the dramatic transformation that has seen Mattek rocket from 153 in the world rankings to 69th place? Put it down to a bucketful of tears, an encounter with a shark, and an extreme form of yoga.

Mattek made her crucial career-defining move following last year's US Open, which she exited in the second round suffering from a form of tendinitis.

She recalled: "It just wasn't enjoyable for me to play. I couldn't run."

Mattek made the brave decision to move from her parents' home in the American tennis heartland of Florida to Arizona, where she hooked up with trainer Jay Schroeder who got her working out to a form of yoga called Iso Extremes.

"You hold positions for like five minutes at a time," she said. "It's really low impact, but you're training at 100 per cent all the time."

Of course, it is one thing being in physical shape, it is quite another making full fitness tell on court. Mattek hit rock bottom earlier this season when she was crushed to a double-bagel by Sharapova at the Family Circle Cup in Charleston.

Devastated, she took herself off to a friend's house where she sat on a bench and "just started crying".

She added: "It wasn't so much losing 0 and 0, but not even showing up for the match. I wasn't intimidated. I don't get intimidated. I just didn't play well."

Mattek briefly forgot about serves, volleys and baseline rallies. Instead, she read books, enjoyed the sun, and even landed a small shark on a fishing expedition.

When she picked up her racket again, she began reeling in results. After acquitting herself rather better against Sharapova at Roland Garros, Mattek reached the semi-finals on grass in Birmingham, beating world No19 Nicole Vaidisova of the Czech Republic on the way.

Now she's making moves in SW19, she said: "This is the most consistent I've ever played. Before, I played a really aggressive game and I would find myself letting a few points or games go by. This year I've really focused on not letting that happen and if I found myself in a funk, just to take my time between points and really think about my strategy, then play the point."

Fashion has made way for form and Mattek admits: "I get other players coming up to me and they're all disappointed [about the lack of outrageous couture]. But now I'm focusing on my tennis.

"I'm still outgoing, I like to show my personality. Off the court I like to dress really funky and creative. But, you know, I really want to have my tennis come through. That's why I've toned down some of my outfits.

A focused Mattek is not the only problem confronting Serena. This fortnight she has already played on Court Two, beating Polish teenager Urszula Radwanska in the second round. But that doesn't mean the sixth seed has escaped the Graveyard jinx.

She herself fell here to 85th ranked Jill Craybas in 2005.

The following June sister

Venus, then the defending champion, became a Court Two victim when she was beaten by 26th seed Jelena Jankovic.

By the end of today, the Williams sisters will have had their fill of a show court that also claimed the likes of Pete Sampras, Andre

Agassi, Pat Cash and Jimmy Connors.

Venus, again the defending title holder, was first up there against 18-year-old Russian Alisa Kleybanova. Then, after Serena's clash with Mattek, the Williams's were scheduled to team up in a doubles match against Anabel Medina Garrigues and Virgina Ruano Pascual of