Haas, who left school less than a year ago and entered the tournament as a wild card, beat the Spanish clay-court specialist, Alberto Berasategui, the French Open runner-up three years ago, 2-6, 6-3, 6-3 to the delight of 12,000 local supporters in the Rothenbaum stadium.
On Wednesday, the 123rd-ranked Haas beat the third-seeded Spaniard, Carlos Moya, the Australian Open finalist, in straight sets and even Boris Becker has been impressed.
"He has the potential to get into the top 50 or even higher - very soon," Becker, who has helped train Haas, said.
Haas, based at Nick Bollettieri's tennis academy in Florida, played a remarkably mature game to overcome both his nerves and a fast-moving baseline opponent with a great deal of experience on clay.
"I was very, very nervous, as this was a new experience and I had no chance in the first set," Haas said. "But it was great to have such support and when I felt more confident I just tried to give it all I had." Haas' reward is a semi-final against another Spaniard, Felix Mantilla, who beat Becker in the third round.
n Great Britain's women were promoted to the Europe-Africa Group One of the Fed Cup in Antalya, Turkey, yesterday. The British, who had won their first four matches this week, beat Denmark 2-1 in their final pool A match.Reuse content