Roger Federer extended his grass-court winning streak to 20 matches as he beat Denis Kudla in straight sets to reach the final of the Halle Open.

The Swiss, who needs to win a 10th title at the German event to retain his world number one ranking, won 7-6 (7-1) 7-5 against the American qualifier.

Federer saved five break points in the second set before breaking in the 11th game.

He will face Croatia’s Borna Coric in Sunday’s final.

• Djokovic to face Cilic in Queen’s final
• Murray drawn against Wawrinka at Eastbourne

Coric’s semi-final opponent, Roberto Bautista Agut of Spain, retired injured when leading 3-2 in the first set of their semi-final on Saturday after he slipped on the grass.

Federer, who skipped the clay-court season including the French Open, began his grass season last week by winning the title in Stuttgart.

The 20-time Grand Slam champion’s form going into Wimbledon is ominous for any player thinking they can stop him from claiming a ninth title at the All England Club.

“I’m very happy, I’ve never played back-to-back finals on grass in the Wimbledon run-up,” Federer said.

“This is a first for me.”

• Live scores, schedule and results
• Alerts: Get tennis news sent to your phone