Roger Federer has pulled out of the Gerry Weber Open after his French Open final loss to Rafael Nadal.
Federer has used the Halle event as his warm-up to Wimbledon
The Swiss star, who has won the last four tournaments in Halle, was beaten 6-3 4-6 6-3 6-4 at Roland Garros.
"In an attempt to prevent injury after the long final in Paris, I cannot defend my title," he told his website.
Among those to make progress on day one in Halle were third seed James Blake, eighth seed Marcos Baghdatis and fourth seed Tomas Berdych.
Federer's four wins in the German grass-court tournament have all preceded similar success at Wimbledon, which gets under way on 25 June.
Sunday's defeat ruined Federer's hopes of capturing a first French Open and becoming only the third man to hold all four Grand Slam crowns at the same time.
Thigh trouble and general fatigue were cited as reasons for his decision to take a short break.
Tournament director Ralf Weber said: "We were informed that he had been bothered by adductor problems in the final against Nadal and that he does not want to take any risks.
"He will return to Zurich, take a few days off, get treatment and then travel to London."
Federer is likely to be fined £20,320 under standard ATP rules if he does not undertake a medical in Halle before Wednesday.