Carlos Alcaraz has left two tennis stars divided following his latest defeat in Doha.

The world No. 3 led Jiri Lehecka in the final set of their quarter-final match before losing the last four games in a row, handing Lehecka the victory.

Retired American tennis players Steve Johnson and John Isner are now at odds over a glaring hole in the Spaniard’s game.

Alcaraz has become known for his lapses of concentration in matches. The same happened during his second-round match in Doha, where he suddenly lost the second set to Luca Nardi after leading.

The 21-year-old managed to recover and won in three sets. But, in the next round, Alcaraz blew another lead and didn’t have time to claw his way back as he suffered a 6-3 3-6 6-4 defeat.

The Spaniard defended himself after the loss, claiming Lehecka simply stepped it up “Honestly, I don’t know what I could have done better,” he said. “I have to give credit to him as well, because when he was down, especially in the third set, he didn’t give up.”

But former world No. 21 Steve Johnson isn’t buying Alcaraz’s excuse. The retired star has grown concerned about the Spaniard’s tendency to “fluctuate” in matches.

Comparing Alcaraz to the top players who came before him, Johnson suggested that the likes of the Big Three, Andy Murray and Stan Wawrinka always finished off the job when they had a lead.

“We just haven’t seen that with conviction from Alcaraz over the last couple of years,” Johnson said on the Nothing Major podcast.

“I believe his top-end tennis is better than anyone when he is playing his best but his mid-range, every day, put on your hard hat and go to work can fluctuate from time to time. It showed, especially in this match.

“He did a really good job, wasn’t playing his best tennis, has a 4-2 lead in the third to go up double break and then, about seven and a half minutes later, he has lost serve twice and the match is over. It is something we haven’t seen from those top guys.

However, former world No. 8 John Isner believes Alcaraz will overcome his concentration lapses as he gets older.

The 16-time title winner replied: “I don’t think this is what we are going to see for the next five to eight years. It is easy to forget that he is still only 21 years old. His best tennis is, you can argue in all likelihood, is five years from now.

“He hasn’t even reached his peak. He is still so young and has so much maturing to do even though he is so ridiculously good and has won all these Grand Slams already at 21 years old.”

But Isner is still expecting Alcaraz to take some tough losses while he matures. “I think he will shore that up and become more consistent,” he continued.

“It is crazy to think that he has not even come close to reaching his highest level as a tennis player.

“This year and maybe next year, he might have some inconsistent results and some losses which make you scratch your head a bit like Lehecka in Doha, but I think he will fix that and will be fine going forward.”

Alcaraz will be hoping to show some more consistency at the upcoming Masters 1000 in Indian Wells, where he is a two-time defending champion.