Serbia's Novak Djokovic serves the ball to Radek Stepanek from the Czech Republic during the quarterfinal of the ATP Qatar Open tennis competition in Doha on January 5, 2017. [Photo/VCG]

Ageless Czech Radek Stepanek's run at the Qatar Open was ended by defending champion Novak Djokovic on Thursday and fellow veteran Ivo Karlovic also bowed out at the quarter-final stage.

World number one Andy Murray, bidding for a third Doha title, joined Djokovic in the semi-final with a hard-earned victory over Spain's Nicolas Almagro, his 27th consecutive win.

Stepanek, at 38 the oldest player to reach an ATP quarter-final since Jimmy Connors in 1995, was no match for a clinical Djokovic, who made light of the breezy conditions with a straightforward 6-3 6-3 victory.

The 37-year-old Karlovic, the oldest top-20 player since Ken Rosewall in 1978, lost 6-2 7-5 to Spain's Fernando Verdasco.

Verdasco will face Djokovic for a place in the final.

Briton Murray beat Almagro for the fifth time in six matches but was never comfortable against the hard-hitting Spaniard before posting a 7-6(4) 7-5 victory.

Almagro threw everything at the 29-year-old, striking 32 winners to his opponent's 24, but Murray's defensive skills proved decisive in the end as he wore down the world number 44.

Murray will play Tomas Berdych, victor over France's Jo-Wilfried Tsonga 7-5 6-3, in Friday's semi-finals.

Andy Murray returns the ball to Nicolas Almagro of Spain (not seen) during the men's singles quarter final match of the 2017 Qatar ExxonMobil Open Tennis Tournament at Khalifa International Tennis and Squash Complex in Doha, Qatar on January 5, 2017. [Photo/VCG]