Halle Westfalen - Roger Federer on Monday flew to Halle to start his second consecutive week on grass but will have to fight for a second straight German title if he is to enter Wimbledon as top seed.
The weekend winner of the Stuttgart Cup - who owns nine titles in Halle - was welcomed with a cake and ceremony by tournament officials as he prepares to start in the first round against Aljaz Bedene.
The 20-time Grand Slam champion moved back to the top spot through his Stuttgart semi-final place.
But to stay ahead of rival Rafael Nadal he must now win the Halle trophy according to ATP calculations.
That's just fine for the Swiss, confident in his game after claiming his 98th career title on Sunday over Milos Raonic.
"I'm always excited for a challenge," Federer said.
"I'm up for it. I'm the holder here anyway, so my goal has always been to defend the title.
"That might take my mind off of number one a little bit; I have hopes of going a long way this week.
"I want to start out of the blocks quickly. But I also want to enjoy being number one again (sixth time). I don't know how long I'll be there.
"A good step would be to defend the trophy and help myself stay on the top ranking. I did a double-take this morning when I looked at the rankings and saw myself at number one.
"It was a cool moment I'd like to stay there."
Japan's Kei Nishikori had to fight through a tiebreaker to start his grass season with a win as he beat Matthias Bachinger.
The seventh seed who has twice played semi-finals at the event, worked for just under 90 minutes to beat his 159th-ranked German opponent 6-3, 7-6 (7/3) and reach the second round.
Yuichi Sugita made it a Japanese double into the second round, beating wild card Maximillian Marterer 6-4, 5-7, 6-3 after two and a quarter hours while holding off a dozen aces from the German.
Nishikori, a former top 10 regular now ranked 27th after wrist injuries last season, was joined as a winner by fourth seeded Spaniard Roberto Bautista Agut, who beat the second German on the day, putting out Jan-Lennard Struff 6-4, 6-1.
"It's never easy but I think I played a very solid, decent match," Nishikori said. "He can be dangerous on grass, he came to net well.
"If he had won the second set, it could have been tight, you never know what can happen in the third. I'm happy with how I'm playing and looking forward to the next round.
"Everything is working well but I cannot expect to much coming back from injury. I'm just glad to be back at this level again."
Dutchman Robin Haase provided early drama with his 4-6, 7-5, 6-4 comeback victory over Portugal's Joao Sousa.
Roger Federer, who won in Stuttgart on Sunday, returned to Germany after a rapid visit to Switzerland.
The 20-time Grand Slam winner raced home by car Sunday night from southern Germany to watch Switzerland draw Brazil 1-1, then flew to Halle on Monday to prepare for his opening match.
The 36-year-old took back the top ranking form Rafael Nadal for the second time this season on Monday, but, according to the ATP, has to win a record 10th Halle crown if he is to enter Wimbledon on the top world ranking.