Top seed Roger Federer got off to a winning start on his Mercedes Cup debut when he defeated 18-year-old Next Generation star Taylor Fritz 6-4, 5-7, 6-4 for a quarter-final clash against Florian Mayer.

"It wasn't easy," said Federer. "I thought Fritz had some good moments there. He gained control of the baseline, he had a little bit of an upper-hand at times. He served well and I was a little bit cautious, unsure of how to move and questioning my defence. These are the things I will need to iron out.

"I can take a lot of confidence out of this match. I need to improve a few things, but I served big and I was able to handle three sets over two days, and find some energy at the end of the match. I know I have an extra gear. My next goal is to stretch leads - once I get a lead, I need to know that I can protect it and then take the next step."

The match resumed on Thursday with Federer leading Fritz 4-3 in the first set, following a rain suspension overnight. Fritz, who had previously played two Top 10 players, converted his fourth set point opportunity in the 12th game of the second set.

"It's always disappointing to take a loss, when you've come close and you've definitely had chances in the match," said Fritz. "I played great and if you'd told me before that I would have taken a set off Federer, I would have been happy."

Federer, who is playing his first match since an Internazionali BNL d'Italia third-round exit to Dominic Thiem on 12 May, has now drawn level with Ivan Lendl on 1,071 victories, for the second-most match wins in the Open Era (since May 1968). Jimmy Connors is the overall leader with 1,256 victories.

OPEN ERA MATCH WINS LEADERS

Earlier in the day, third seed Thiem recorded his first victory as a Top 10 player when he withstood 21 aces from Sam Groth in a 7-6(3), 7-6(2) win over one hour and 31 minutes. The 22-year-old Austrian moved from No. 15 to No. 7 in the Emirates ATP Rankings after a Roland Garros semi-final run (l. to Novak Djokovic).

"It was an unbelievably tough and close match," said Thiem. "I expected it. I played two very good tie-breaks, which was the key. He is a very tough opponent on the grass."

Thiem will next take on Mikhail Youzhny, who knocked out qualifier Sergiy Stakhovsky 7-3, 7-6(1) in 81 minutes.

Radek Stepanek, a 37-year-old qualifier, lost just four of his first service points to edge out second seed Marin Cilic 7-6(4), 7-5. He last reached the Stuttgart quarter-finals in 2004 (l. to Canas) and today's result represents his first ATP World Tour quarter-final since July 2015 in Bogota.

Fourth seed Gilles Simon lost only eight of his first service points to advance with a 6-4, 4-6, 6-2 victory over Jan-Lennard Struff in one hour and 41 minutes. Wild card Juan Martin del Potro booked a spot in his first grass-court quarter-final since reaching the 2013 Wimbledon semi-finals. The Argentinean beat John Millman 6-4, 6-4.