ROTTERDAM (AFP) - Ernests Gulbia ended Juan Martin del Potro's winning run when he beat the top seed 6-3, 6-4 in a quarter-final upset at the Rotterdam World Tennis on Friday.

Gulbis, the world number 24 from Latvia, will face third seed Tomas Berdych in the semi-finals after the Czech defeated Poland's Jerzy Janowicz 6-7 (9/11), 6-2, 6-4 in a hard-fought victory which took nearly two-and-a-half hours.

In his last five tournaments as the top seed, defending champion Del Potro had won five titles and 23 straight matches.

Wimbledon winner Andy Murray was trying to maintain the honour of seeds as the number two faced former top 10 competitor Marin Cilic of Croatia in the quarter-finals.

German Philipp Kohlschreiber was playing the last surviving Dutchman in Igor Sijsling.

Gulbis, who can be brilliant or less than impressive depending on his mood, preyed upon Del Potro as the Argentine took treatment before the start of the second set on the left wrist which has been posing problems since last month in Australia.

Del Potro came to the match without much ability to hit a penetrating backhand as he plays with medical permission while trying to clear up his wrist problem.

Gulbis will be playing in his first semi-final at the event after advancing in 81 minutes with a dozen aces as he forced Del Potro to save seven of nine break points.

"It was a tough match mentally I had to stay in control," Gulbis said. "I'm really happy to reach the semi-finals. Both Tomas and I will be trying everything in the next round." Gulbis won his second match out of five against Del Potro, claiming the opening set in 36 minutes with a break in the penultimate game after the Argentine had saved three chances of a break.

After Gulbis won it 6-3, Del Potro then had his wrist taped, a sure sign that his discomfort level was rising.

Gulbis took full advantage in the second set with a break for 2-1 from a lob winner.

The Latvian was unable to convert on two match points against the Del Potro serve in the ninth game, with the Argentine saving them both to stay in touch, 4-5.

But Gulbis, who won the last of his four career titles in September at St Petersburg, was not to be denied.

The Baltic challenger stepped up again two games later on his own serve, missing on a third match point but saving a break point with an ace.

He finally secured victory with a volley winner at the fourth attempt.

Berdych said he needed mental strength to get past Poland's big-hitting Janowicz.

"It was a very tough one for me," said the world number seven, playing the event for the eighth time.

"Staying mentally strong was my biggest accomplishment. Winning it - even in three sets - was the most important thing.

"I had to really fight today. it was my mental part that I'm most proud of." Berdych leads Gulbis 3-2 with their last meeting a win by the Czech in Marseille a year ago.