Australian duo John Millman and Sam Groth are out of the Mercedes Cup in Stuttgart after losing in the second round on Thursday.

Wild card former US Open champion Juan Martin del Porto, coming back from a wrist injury, beat Millman 6-4 6-4.

Third-seeded Dominic Thiem edged out Groth 7-6 (7-3) 7-6 (7-2) in the battle of the big servers.

In his first match in a month, Roger Federer overcame a stiff challenge from American teenager Taylor Fritz to reach the quarterfinals with a 6-4 5-7 6-4 win.

The match began the day before but was halted by rain with the top-seeded Federer leading 4-3 with a break of serve.

A return into the net by Fritz gave Federer the first set. Federer saved two set points before a forehand winner by Fritz and a forehand error by Federer gave the middle set to the 18-year-old American.

Federer struggled to hold serve early in the third but broke for 5-4 and clinched the match with a backhand winner.

“It was very difficult, just as I knew before the match,” Federer said. “I was more fortunate at the end.”

A back injury forced the 34-year-old Federer to skip the French Open, the first time he missed a Grand Slam after 65 straight appearances.

“After such a long injury, you need to find your rhythm,” Federer said.

It was Federer’s 1,071st career win on the tour, tying him with Ivan Lendl. Only Jimmy Connors has more, with 1,256.

The Stuttgart tournament is on grass as a tuneup for Wimbledon, which Federer has won seven times.

Now ranked No. 3 in the world, Federer is next up against Florian Mayer.

While Federer may have been rusty, second-seeded Marin Cilic was completely off, losing to the veteran qualifier Radek Stepanek 7-6 (7-4) 7-5.