Marcos was forced to retire in the second set of his 2017 opener in Doha. Trailing qualifier Radek Stepanek 6-4, 5-3, he pulled out due to a right ankle ailment.
Marcos was making his first appearance at the Qatar ExxonMobil Open since 2007, when he reached the quarterfinals. He was also facing Stepanek for the first time in nearly six years.
Marcos will look to be fit for his doubles debut on Tuesday, partnering with Marcin Matkowski against top seeds Jamie Murray and Bruno Soares. His next tournament is scheduled to be in Auckland, followed by the Australian Open.
