CINCINNATI, Ohio (AFP) - Novak Djokovic shelved any lingering sentiment as he dispatched a longtime friend to book a place in his fifth Masters Series semi-final of the season on Friday.

The third-seeded Serbian stopped Latvian talent Ernests Gulbis 6-3, 6-4 in the second battle in three months between one-time tennis camp rivals.

The young pair - at 21 Djokovic is two years older than Gulbis - spent time at the Niki Pilic academy in Munich and fondly remember their tearaway teenaged years.

"It was a matter of time when he was going to make this breakthrough and just gain the experience with the time," said Australian Open winner Djokovic. "Obviously he's showing his quality now.

"He's one of the young rising stars that we expect to see more in the future."

Djokovic rode one break per set to improve his record over Gulbis to 2-0.

He awaits a chance at Rafael Nadal in the final four if the Spanish second seed continues the red-hot pace which is taking him towards the long-sought number 1 ranking.

Nadal had a later quarter-final against Nicolas Lapentti of Ecuador.

At the top of the draw, Andy Murray beat Carlos Moya 2-6, 6-3, 6-1 to complete a spirited fightback.

The eighth-seeded Scot will test his confidence against the mighty serve of Ivo Karlovic, who notched 24 aces in his 7-6 (8/6), 7-6 (7/3) win over German Philipp Kohlschreiber.

On Thursday, the Croatian giant greased the skids for Roger Federer to lose his number 1 ranking, stunning the Swiss in the third round.

Nadal can clinch the top spot on Monday by winning Sunday's final.

Murray's success took just over 90 minutes as the eighth seed turned around a desperate situation, down a set and a break in hot summer conditions.

The Briton goes in with full confidence after reaching his fifth career semi-final at the elite Masters level.

"I think I can win a Masters Series, and I'd love to do it this week," he said. "I played well last week and had a tight match against Nadal.

"This week I'm playing well again. You know, hopefully I can do it."

Murray improved his season mark to 33 wins, 12 defeats.

"It was really tough at the start," he said. "He was serving well and I threw in a lot of unforced errors. But once I got going, I played really well.

"I've never been to the final of a Masters but this looks like my best chance. I'm playing well enough and can hopefully come through."

Moya, at 31 a decade older than Murray, ran out of puff after his initial fast start, failing to put Murray down a set and two breaks and then fading away in the third set.

"To go two breaks down, a set and two breaks down, would have been tough to come back from," Murray said. "But it happens sometimes in tennis."