Federer into Stuttgart semi-finals

Switzerland's Roger Federer returns the ball to Argentina's Guido Pella in their quarter-final match at the ATP Mercedes Cup tennis tournament. (THOMAS KIENZLE/AFP)

The Swiss top seed needs to win one more match against either Australian fourth seed Nick Kyrgios or 2017 finalist Feliciano Lopez to return to the number one spot for the second time this season.

"I didn't think of number one during the match today, I was focused on the task at hand," said Federer. "But when you play for number one (next match) I'm sure it will be in the back of my head."

The quarter-final victory in 55 minutes, which ended on a backhand, featured two breaks of serve for the Swiss, who executed well in his second match in three months after skipping the clay season and last playing in March.

"I was in good control throughout," added Federer.

"I served well and didn't give him too many chances to get into my service games.

"I thought I was able to stay aggressive, and cut down the length of the points, which I wanted to do."

Federer said that he was able to train briefly on the grass court of a friend in Switzerland but said that fitness work was the major part of his preparation after his absence from the clay court campaign.

"We used to have a shorter turnaround from clay to grass. Now we have many weeks to get ready (three weeks between the end of the French Open and start of Wimbledon).

"I was lucky to be able to get a few days on the grass before here, that was helpful."

Federer is aiming for a first title in Stuttgart after losing in the 2016 semis to Dominic Thiem and going out in an opening match a year ago to close friend Tommy Haas.

The 36-year-old won his 19th match from 21 played this season as his Wimbledon buildup continues on track.

At the All England club next month, the 20-time Grand Slam champion will be seeking a ninth title at the grass-court major.

At the bottom of the draw, Milos Raonic reached his second semi-final of the season to set up a clash with defending champion Lucas Pouille.

Raonic used his big-serving game to dominate a pair of tiebreakers in a 7-6 (7/2), 7-6 (7/1) defeat of Tomas Berdych.

"I got fortunate in the tiebreakers," the 27-year-old Canadian seventh seed said after reaching his first semi since Indian Wells last March.

The often-injured Raonic said that when he's healthy, he likes his game; he played just six matches from the end of Wimbledon last summer until the start of 2018.

"I can play well when my body allows. I wasn't ready for Australia (last January). But now that I'm back I'm playing well," said the player now coached by former great Goran Ivanisevic.

Raonic has beaten Pouille in both of their previous matches, both played in January, 2016, in Australia.

The French second seed overcame a nervous second-set to reach the semis 6-4, 6-7 (5/7), 6-3 victory over Uzbek Denis Istomin.

Quarter-final results:

Roger Federer (SUI) bt Guido Pella (ARG) 6-4, 6-4

Nick Kyrgios (AUS) bt Feliciano Lopez (ESP) 6-4, 3-6, 6-3

Milos Raonic (CAN) bt Tomas Berdych (CZE) 7-6 (7/2), 7-6 (7/1)

Lucas Pouille (FRA) bt Denis Istomin (UZB) 6-4, 6-7 (5/7), 6-3