Sampras committed an unthinkable 54 unforced errors on Wednesday and is in danger of losing his world No 1 ranking this week. On a cool and windswept night, the top seed struggled with both the mental and physical aspects of his game before falling to Mantilla 7-6, 3-6, 6-3.
"I was lacking a little confidence. I didn't feel right all night," admitted Sampras, who has played only six matches this year. "It was cold and the ball was heavy. I struggled and just didn't play well. I couldn't get my game going."
Sampras, who was unable to unleash any kind of effective chip-and-charge attack in the two-hour match, added: "It's back to the drawing board. I'm going to take this loss very hard. It's a wake-up call."
The departure of Sampras left another Spaniard, Carlos Moya, in position to seize the top ranking after the fourth seed repelled a strong challenge from Jim Courier, a former No 1, to score a 7-6, 6-4 victory.
At the start of the tournament, the second-ranked Yevgeny Kafelnikov, the No 3 Alex Corretja, Moya and the fifth-ranked Patrick Rafter all had a chance to unseat the suddenly vulnerable Sampras this week.
Kafelnikov and Rafter lost their opening matches on Tuesday. Corretja, up a set and a break on Wednesday, blew a 4-1 lead in the second set against Australian Mark Philippoussis and eventually fell 4-6, 7-5, 6-2.
The three-times Wimbledon runner-up, Goran Ivanisevic, the 14th seed, also was bitten by the upset bug. He was defeated by the 18-year-old Belgian, Xavier Malisse, 7-6, 6-3.
Playing in only his second event this year, the sixth seed and defending champion, Marcelo Rios, who has been nursing a back injury, was impressive in taking apart France's Cedric Pioline 7-6, 6-1. The Chilean won this event and the Lipton Championship back-to back last year and briefly became No 1 in the world.
"I've been feeling pretty good after four months when I really haven't played," Rios said. "Maybe my back is bothering me a little when I serve, so I don't have the power. But there's no more pain."
Other players through to the quarter-finals yesterday were Gustavo Kuerten of Brazil and the eighth-seeded Dutchman Richard Krajicek. Kuerten beat Sjeng Schalken, of the Netherlands, 6-2, 6-3, while Krajicek sank his compatriot Jan Siemerink 6-4, 6-4.
In the Evert Cup, the women's tournament being played alongside the men's event here, Steffi Graf continued her recovery last night by overcoming Chanda Rubin to reach the final.
The 26th-ranked Rubin had claimed the biggest scalp on offer, by picking off the top seed and defending champion, Martina Hingis, 6-3, 7-6, on Wednesday night.
Graf had reached the semi-finals by crushing Jana Novotna, 6-2, 6-0, for the 29th win in her 33 career meetings with the Czech serve and volleyer. Leading 4-2 in the first set, Graf raced through eight consecutive games to end the match in a swift 52 minutes.Reuse content