Grand Slam champions Dominic Thiem and Stan Wawrinka both made their comebacks from long-standing injuries in Marbella, but both suffered defeats.
Thiem, who won the 2020 US Open in dramatic fashion when he came back from two sets down to beat Alexander Zverev in a deciding set tiebreaker, had not played since June sue to a wrist injury.
He face world number 228 Pedro Cachin in the Marbella challenger event, but showed his rustiness as he lost the first five games of the match.
The Austrian managed to steady himself but ultimately lost 6-3 6-4 to cut his comeback short.
"Today's been one of those days with big emotions and mixed feelings," Thiem said afterwards on Instagram.
"You can imagine how happy I was today to be on a tennis court, doing what I love the most: playing tennis and competing again.
"After all these months, all those setbacks I had in the past few months, I was able to play again to feel the adrenaline of the competition. I felt like a player again."
Stan Wawrinka did not have any more luck, losing 6-2, 6-4 to world number 131 Elias Ymer.
Wawrinka, a three-time Grand Slam win, had not played for more than a year since losing to Lloyd Harris at the 2021 Doha Open.
He has since undergone two surgeries on his foot, and it now appears a long-way back for the 36-year-old, who has slipped down to number 232 in the ATP world rankings.