They may have 43 grand slam titles between them but it didn't help on Tuesday as Roger Federer and Serena Williams both suffered shock losses to leave the tennis world stunned.
In his first outing in two months, Federer lost 6-4, 4-6, 6-4 to Spanish clay-court specialist Pablo Andujar in Geneva to wreck his hopes of stringing a run of matches together ahead the French Open, Wimbledon and the Tokyo Olympics.
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And less than an hour later, Williams' disappointing return to tennis continued when she lost in straight sets to 68th-ranked Katerina Siniakova in the second round of the Emilia-Romagna Open.
Federer's loss was particularly shocking given it marks his first defeat on home soil in seven and a half years.
The 39-year-old tentatively returned to the courts in Doha in March, having been out for more than a year following two knee surgeries, winning his first match before losing his second.
The 20-time grand slam champion was hoping to find some form on the Geneva clay but tumbled at the first hurdle on Tuesday.
"It's good to be back on the court but then you lose a match like this and you're down," Federer told reporters.
"I know my limitations at the moment.
"People expect a lot from me and I have high expectations for myself.
"So when I walk out of a match and feel I could have played so much better, it feels strange and it's disappointing."
Federer said he would now work on analysing his game to identify where he's falling short.
"This is the process I need to go through. I can't get too down on myself," he said.
"I need to go back to the drawing board and talk to the team and say 'what's the plan here for the next 10 days?'.
"What did they see from the outside that maybe I didn't see and feel? There's going to be a lot of conversations."
Serena Williams follows Federer with shock loss
Fellow 39-year-old Williams, who accepted a wildcard invitation for the Parma tournament after losing her opening match at the Italian Open last week, was beaten 7-6 (4) 6-2.
She had beaten teenage qualifier Lisa Pigato 6-3 6-2 in the previous round for her first victory since defeating Simona Halep in the Australian Open quarter-finals in February.
After some time off, Williams then lost in straight sets in Rome to Nadia Podoroska.
The 23-time grand slam champ is preparing for the French Open in Paris which starts on May 30.
"I didn't do a lot of mistakes, and I think she felt it," Siniakova said.
"I think she was trying to go harder and doing a lot of mistakes, which helped me. I'm really happy I could keep my level all match."
Williams racked up double faults under pressure and Siniakova sealed the result on her third match point when the American hit a forehand long.
Williams had seven double faults for the match.
"It's amazing, it means a lot to me," Siniakova said after the sixth Top 10 victory of her career.
"I played (an) amazing match. It shows me that I can play like this, I can play well. I just need to try be more focused every time and to stay calm and ready for every point. I will be really happy if I could keep it like this."
Tennis fans were left stunned as Federer and Williams both lost within hours of each other.
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