Andy Murray's latest comeback ended after he was outplayed by Russia's Daniil Medvedev in the second round of the Brisbane International.
The Briton, in his first tournament since September, pushed Medvedev before the world number 16 took control at the end of the first set.
Murray, 31, looked to wane as the 22-year-old continued to overpower him and win 7-5 6-2 in one hour and 21 minutes.
The Scot is ranked 240th in the world after playing six tournaments in 2018.
"When you play better players they will expose any errors that you make in your game or any shots that you're not hitting particularly well. And he did that," Murray said.
After his opening round win over Australian wildcard James Duckworth, Murray admitted he did not know how much longer he would be able to play top-level tennis after hip surgery last January.
His next scheduled tournament is the Australian Open - the first Grand Slam of the year, which starts on 14 January - although he could decide to take a wildcard for next week's Sydney International.
Questions still remain about the level Murray will be able to reach following hip surgery a year ago, with many fans wondering if his game can ever return to the same standard that made him one of the world's best players.
His latest comeback - his first tournament since losing at the Shenzhen Open in September - started with a comfortable win over Duckworth, but Medvedev was always going to provide a sterner test.
Murray competed well in the opening set against a player regarded as one of the rising stars on the ATP Tour, but his lack of fitness and match practice eventually showed, understandably, in the early stage of another return.
The former world number one proved his fighting qualities have not diminished, battling back from dropping serve in the third game to level again for 4-4.
However, Murray lost serve for 6-5 in the first set - and that proved decisive.
Fourth seed Medvedev held serve to wrap up the opener, then raced into a 4-0 lead in the second as Murray appeared to start struggling with his movement.
Murray, a two-time champion in Brisbane, rallied to hold serve twice but did not look like making a serious impact on his younger opponent's serve.
Medvedev continued to control the match, making only three unforced errors in the second set, going on to take the second of three match points with his 16th ace.
"He served extremely well and I didn't really get many chances on his service games," Murray said.
"And then on my own service games I made a few too many errors in the rallies, especially early on - I was trying to play a little bit more offensive.
"In the second set, I made a few too many mistakes, and obviously there was a period in the match from 5-5 where I lost six games in a row."
Murray, one of the most popular players on the tour, looked upset as he waved to all sides of the stadium before leaving the court to a standing ovation.
"It is always tough against Andy," Medvedev said. "I don't know what ability he is at now but he is one of the champions of our sport so it is never easy."
Murray's exit ended British interest in Brisbane, with his defeat coming shortly after Kyle Edmund - the man who replaced him as British number one - also lost.
Edmund, the world number 14 and third seed, suffered a shock defeat by Japan's world number 185 Yasutaka Uchiyama, who won 7-6 (8-6) 6-4.
The Yorkshireman, who had a bye in the first round, failed to convert two set points in the opening set against the qualifier.
He was broken to love in the third game of the second set and Uchiyama went on to wrap it up quickly.
In the women's event, Johanna Konta's campaign also ended in round two after defeat by Australia's Ajla Tomljanovic, while British number four Harriet Dart lost 6-2 6-0 to Latvian world number 11 Anastasija Sevastova.