Roger Federer is competing in his first ATP tournament since his third round defeat to Dominic Thiem at the Internazionali BNL d'Italia in Rome.
Federer opted to skip the French Open as he was still hampered by a back injury, and the world number three hasn't been used to having plenty time off the tour due to injury.
After his four set semifinal defeat to world number one Novak Djokovic at the Australian Open, Federer missed a few months on tour due to having a knee injury which required surgery.
It certainly would have been hard for Federer to sit out and miss the French Open which ended his remarkable run of 65 consecutive Grand Slam appearances.
Djokovic's triumph at the French Open would have made it even harder for Federer to watch as Djokovic is now on 12 Grand Slam titles and completed his career Grand Slam and he is closing in, on Federer's 17 Slams.
Thiem is in a good vein of form coming into this intriguing semifinal clash with Federer at the Mercedes Cup as Thiem played lights out tennis during the clay court season which culminated in a semifinal loss to eventual champion Novak Djokovic at the French Open.
The Austrian's amazing run at the French Open made him crack the top ten for the first time in his career at number seven.
How they got here
Both players received byes in this tournament as they are seeded first and third respectively.
Federer's first match back on the tour was against ATP Rising Star Taylor Fritz and the match lasted over two days as Mother Nature got the better of proceedings on the court.
It was understandably a rusty performance by Federer but he came through that test in three sets, defeating the young American 6-4, 5-7, 6-4.
Federer's quarterfinal match against German qualifier Florian Mayer was a familiar outcome for the Swiss maestro as the 17-time Grand Slam champion defeated the German 7-6, 7-6 for the seventh time and for the fifth time in Germany in their meetings.
Thiem's passage to the semifinals was similar to Federer as he defeated big-serving Australian Sam Groth 7-6, 7-6 in his first grass court match and in the quarterfinals he defeated former world number eight Mikhail Youzhny from a set down to record a 3-6, 6-4, 7-5 victory.
Their history
Federer and Thiem have played each other twice and both of their meetings took place this year and their head-to-head is level at one win apiece.
Federer won their first meeting in Brisbane at the semifinals on hard courts which he won 6-1, 6-4.
Their most recent meeting happened to be Federer's last tournament prior to this one and it was in the third round stage of the Rome Masters which Thiem won 7-6, 6-4.
During their match on the clay courts at the Foro Italico, it was clear that Federer was clearly hampered by his back.
Who wins
Thiem has had to make the quick transition from clay courts to grass courts and this surface has always been the most difficult for the young Austrian.
The third seed's best results have come on clay and hard courts and his best result at Wimbledon was the second round last year.
Thiem is competing in his first semifinal on grass and this could be his best opportunity to beat Federer on grass and reach his first grass court final.
Although Federer hasn't been playing tennis in his vintage form, he will be happy to have a couple of matches under his belt and winning the tournament here in Stuttgart would be a bonus for him.
Federer knows that missing the French Open was the right decision for him as it allowed him time to heal his back and not aggravate it.
Playing on grass will give Federer some confidence as he searches for his first Grand Slam title since 2012 and 18th Grand Slam title overall at his beloved Wimbledon.
This semifinal match will take place on the Centre Court at 12pm local time on Saturday and the winner of this match will play Juan Martin Del Potro or Philipp Kohlschreiber in Sunday's final.
Prediction: Federer in three sets.