Rafael Nadal was beaten by Kevin Anderson on his return to action at the World Tennis Championship, where Novak Djokovic reached the final.
The Spaniard endured a tough 2018 season with injury, completing just one hard-court tournament and most recently missing the year-end ATP Finals with an abdominal complaint.
And playing on Friday for the first time since his semi-final retirement at the US Open in September, Nadal suffered a 4-6 6-3 6-4 last-four loss to defending champion Anderson at the Abu Dhabi exhibition.
The 17-time grand slam champion nudged in front midway through the first set and, although Anderson swiftly responded, another break and a hold secured the opener.
But Anderson shook off Nadal in the second to level the match and then remained patient under pressure, the South African breaking to lead 5-4 before closing out victory.
Despite the defeat, Nadal reflected on-court: "I need to take it step by step after my [ankle] surgery, but that's a positive start."
World number one Djokovic awaits Anderson in the final after racing to a 6-4 6-2 win over Karen Khachanov.
The Serbian made a fast start and wrapped up the first set despite subsequently slowing, then found his groove again in the second to break twice and finish with a flourish.