LONDON, June 28: Second seed Serena Williams slopped through a match to forget Friday, slipping and sliding on the grass to a 7-6 (7-5), 7-6 (7-2) win over Belgian Els Callens to reach the fourth round of the Wimbledon Championships.

The American looked like never finding her feet as she escaped a potential upset ambush on the court 2 “graveyard” which claimed seven-time champion Pete Sampras in the men’s second round at the All England club.

French Open winner Williams had a handful of trouble from world number 199 Callens in their first career meeting.

Williams ended up on her backside several times as her feet literally flew out from under her on the short, slick grass, now worn down by five days of play. Commentators suggested she may not have been wearing proper grass-court shoes.

Callens put the second seed off her game with a break in the eighth game of the opening set.

But the 31-year-old promptly lost the edge as Williams broke back a game later for 4-5, then carried on to a tie-breaker, which she won with a 4-0 start.

The second set was a near-copy of the first. Williams, with top-seeded sister Venus looking on, lost serve for 2-3, broke back for 4-all and struggled to finally take the victory through another decider.

Third-seeded Jennifer Capriati managed a more secure decision as the tournament enters the middle weekend, beating Czech Daja Bedanova 6-4, 6-2.

There was a battle for seventh-seeded Yugoslav Jelena Dokic, who overcame French opponent Nathalie Dechy 7-5, 6-2 and next plays fellow pony-tailed blonde Daniela Hantuchova, winner over Maja Matevzic of Slovenia 6-4, 6-4.

Ninth seed Amelie Mauresmo was troubled by a hamstring injury before thundering past Russian Anastasia Myskina 6-4 6-2.

Having taken a three-minute injury time out midway through the first set to get her left thigh strapped, Mauresmo reeled off nine straight games from 3-1 down to take control of the match.

Myskina, who came into the tournament with high hopes after reaching grasscourt finals in Edgbaston and Eastbourne over the last two weeks, offered little resistance in the second as the Frenchwoman dominated from the baseline with her bludgeoning groundstrokes.

Mauresmo will next face American qualifier Laura Granville, who stunned former Australian Open champion and crowd favourite Mary Pierce of France 3-6 6-4 6-1.

Xavier Malisse notched the latest men’s upset of the week, knocking out fifth seed Yevgeny Kafelnikov of Russia 7-6 (7-4), 7-5, 6-1.

Australian Mark Philippoussis blasted 33 aces on his way to a 3-6, 6-3, 6-4, 6-2 third-round win over German Nicolas Kiefer to advance in impressive style.

Philippoussis, a three-time quarter-finalist, ran his total of untouchable serves to 81 over three matches.

The Aussie will face a test of serve in the fourth round against Dutchman Richard Krajicek, the 1996 winner and only former champion remaining in the draw after early exits by Pete Sampras and Andre Agassi

Krajicek advanced against Thai Paradorn Srichaphan 7-6 (7-5), 6-4, 6-2 in his second event back after 18 months away with an elbow injury.

Philippoussis has come good on the grass after a season of dismay in 2001, when he missed six months after multiple knee surgery. He received a wild card as his current standing is too low for a direct entry into the draw.

Fellow Aussie Wayne Arthurs, another fearsome server, booked into the fourth round over American Taylor Dent, winning a 7-6 (7-2), 7-6 (7-3), 6-7 (4-7), 7-6 (7-5) thriller.