Stefan Koubek caused a huge upset, shocking second seeded Mario Ancic of Croatia 3-6, 6-4, 6-3 to storm into the semi-finals of the US $380,000 Kingfisher Airlines ATP Mumbai Open on Friday.

The former top-20 Austrian, now ranked 118, took full advantage of an error-ridden display by the world No 11 to enter the last four stage, where he will meet Tomas Berdych of the Czech Republic.

Third seed Berdych progressed to the semi-finals after a comfortable 6-3, 7-5 victory over Germany's [Images] Bjorn Phau.

Russia's [Images] Dmitry Tursunov [Images] continued to stutter through the tournament, thwarting a strong challenge from qualifier James Auckland. The Russian overcame rough conditions and problems with string tension in his racquet to also advance to the semi-finals with a 3-6, 6-3, 6-4 victory.

Tursunov will meet top seed Tommy Robredo. The Spaniard made it to the semis, scoring a rather comfortable 6-2, 7-6 (7-1) victory over Paraguay's Ramon Delgado in the last quarter-final of the evening.

Semi-finals line-up

Tomas Berdych vs Stefan Koubek

Dmitry Tursunov vs Tommy Robredo

"I carried two racquets and the string on one broke during the match, so I gave it to the stringer; he didn't understand what I wanted but still said yes, and when he came back the string tension wasn't what I wanted," explained Tursunov, who seemed pretty upset about the mix-up.

He also had an argument with umpire Jean-Phillippe Dercq of Belgium on the matter but wasn't allowed to leave the court to sort it out.

"The umpire didn't think it was an urgent matter. While I was getting angry and shouting I was careful not to get too upset and let it affect my game," said Tursunov, who was once seen jumping on the racquet to loosen the strings.

Auckland served big in the first set and approached the net at every possible opening. Tursunov, who hasn't brought his best game to Mumbai, struggled with his rhythm and very often was left stranded on the baseline, allowing Auckland to complete another well-designed point with his backhand volley.

But the Russian, stretched even in his first two matches, came back stronger, breaking Auckland in the very first game of the second set and running to a 3-0 lead. His passing shots started getting some sting now and Auckland was losing the fizz.

Tursunov took the momentum into the third set, and earned the all-important break in the fifth game which was enough to nudge him through into the last four.

Berdych made light work of the small-built Phau and served 10 aces to coast to victory in 72 minutes. The Czech had only one hiccup in the end when he was broken at 5-4 while serving for the match.

But Berdych broke back in the very next game and served out the match with a solid service game.

The Koubek-Ancic match was an engrossing duel, with the players engaged in baseline play. A break in the fourth game saw Ancic take a 3-1 lead and then proceed to claim the set. It raised visions of the Croat cantering away.

But 29-year-old Koubek, who has won three ATP titles, had other ideas. He hit back, breaking the Ancic's serve in the sixth game. And though he dropped serve at 15 in the next game, he had enough firepower to break again in the tenth game and close out the second set.

A set apiece, the stage was set for a battle royal. And neither player disappointed the goodly crowd. Koubek chose to attack the net while Ancic was wary of doing the same. That perhaps proved the later's undoing.

Koubek got the crucial break in the sixth game when Ancic attempted a drop volley that failed to clear the net.

Ancic then saved two match-points on serve in the eighth but was unable to capitalize on a break-point on Koubek's serve in the next game. The Austrian kept a cool head, and aided by some unforced errors from Ancic, took the game and set to seal a place in the semis.

Results:

Men's singles (quarter-finals): 3-Tomas Berdych (Czech Republic) beat Bjorn Phau (Germany) 6-3, 7-5; Stefan Koubek (Austria) beat Mario Ancic (Croatia) 3-6, 6-4, 6-3; Dmitry Tursunov (Russia) bt James Auckland (Great Britain) 3-6, 6-3, 6-4; Tommy Robredo (Spain) beat Ramon Delgado (Paraguay) 6-2, 7-6 (7-1).

• Complete tennis coverage
• All you want to know about Sania Mirza