Marcos got off to a strong start at the Citi Open in Washington, D.C., before succumbing to top seed John Isner on Thursday.
The strong start saw him need just 59 minutes to defeat John Millman 6-2, 6-4, in his opening encounter of the tournament. Marcos fired four aces and saved all four break points faced, while converting on all three earned againt the Aussie's serve. It was his 18th match win of the season.
On Thursday, Marcos would face off against top seed and three-time Washington finalist John Isner. It was their second meeting in less than a month, following a clash on the lawns of Wimbledon in late June. Marcos forced an opening tie-break, but would eventually fall 7-6(3), 6-2 after 70 minutes.
Marcos will next move on to compete at the Rogers Cup in Toronto. It is the sixth ATP World Tour Masters 1000 event of the year. Marcos last appeared at the Canada Masters in 2013.