British number two Kyle Edmund followed world number one Andy Murray out of the Internazionali BNL d'Italia after a straight-sets defeat against Juan Martin del Potro in Rome.
The 22-year-old produced a brave battle against the world number 34, breaking back to level the opening set at 5-5.
However, the experienced Argentinian, US Open champion in 2009, kept the pressure on by immediately forcing another break and then served out to take the first set 7-5.
Edmund's serve continued to be tested in the second set, where the Briton was involved in an heated exchange with the chair umpire over a disputed line call during a tense fifth game, which Del Potro went on to win before moving 5-2 ahead.
Although Edmund, ranked 53 in the world, rallied to break and then hold to reduce the deficit to 4-5, it was only delaying the inevitable as Del Potro, who had defeated world number 11 Grigor Dimitrov in the first round, closed in on victory.
Del Potro, who helped Argentina win the 2016 Davis Cup, spurned two match points before a net return from Edmund saw him progress 7-5 6-4 in one hour and 46 minutes, safely through to the last 16 where he will face Kei Nishikori of Japan, who is seeded seventh.