Witter, 35, had been attempting to win back the WBC title he lost to Timothy Bradley at the Nottingham Arena nearly 15 months ago.
The Sheffield fighter had appeared to be behind on the judges' scorecards at the Agua Caliente Casino Resort Spa but his decision to quit came as a surprise, sending his record to 37-3 with two draws (22 KOs).
Alexander, 13 years Witter's junior, remains undefeated, his record moving to 19-0 (12 KOs).
Switch-hitting Witter had come out fighting in an orthodox but awkward style and he went down in the first round after losing his balance having missed with a wild right hook. When he got up he immediately switched to southpaw.
The second round saw Witter land a left hook to Alexander's head but the Englishman was rocked just before the bell with a short right hook.
Both men landed stiff right hands, Alexander first, at the start of the third round but with Witter's awkward style producing a clash of heads the Englishman emerged with a cut above his right eye and an Alexander right hook hurt him further inside the final minute of the round.

Witter was rocked again in the fifth, getting caught with a straight left on the ropes but managing to hang on to Alexander and staying on his feet.
Witter had been knocked down in the sixth against Bradley on the way to his split decision defeat but he did not come under pressure from Alexander until right before the bell when the American countered a strong right hook from the Briton with a big left as the bell sounded.
Witter started the seventh round well, working the right jab consistently with Alexander offering little in return and finishing with a big left from a southpaw stance.
A tetchy eighth round passed with no major incident, although referee Lou Moret's patience with Witter's regular holding of Alexander started to wear thin.
Moret approached the Englishman's corner at the end of the round and Witter rose from his stool, crossed the ring to Alexander's corner and congratulated the American, stunning Alexander with the news he had quit.
Witter and trainer Dominic Ingle retired immediately to their dressing room as Alexander celebrated, where he was set to be examined by the ringside physician.
A dejected Witter said he had incurred an injury to his right elbow early in the fight.
"My elbow went in round four and basically I couldn't keep him off with my left hand," Witter told Showtime Television. "I battled through for a few rounds but the pressure kept coming on and it was like 'I can't keep doing it'.
"I kept switching, which was working to an extent but I wasn't able to box the way I wanted to. I didn't feel good. I didn't feel good at all and we just decided it was time to let this one go.
"It was a mutual agreement (with his corner). As much as I wanted to do it, I just wasn't able to do it. It's not that I wanted to quit, I wanted to win, I knew it was a close fight, there wasn't much in it, he caught me with some good shots, I caught him with some good shots and I still had a chance to win the fight - but I just wasn't here on the night."
This article has 4 comments
His guy is just like Froch full of hot air coming out of his mouth. Met him once at a Mazda gig we were running and he was so arrogant.
By Toby. Posted November 1 2009 at 3:04 PM.
Junior Witter is the joke of English Boxing!! He has no style! No Heart and NO CLASS!! He shown tremendous envy towards Ricky Hatton and his success and now calling out Amir Khan!! He should do everyone a favour and QUIT!! Walk away from boxing for good!!
By M.codd. Posted August 2 2009 at 8:55 PM.
THIS GUY IS SOOO POOR , ALL WEEK HES TALKING UP FIGHTS AGAINST KAHN SAYING HED KNOCK HIM OUT IN ROUND 3 , NOT EVEN CONCENTRATING ON THE FIGHT AHEAD , JUST A FIGHTER THATS ALL TALK , HE WOULD GET DESTROYED BY KAHN WITH HIS STUPID STYLE .... KEEP TALKING AND DREAMING JUNIOR THE QUITTER WITTER
By RICHARD JAMES. Posted August 2 2009 at 6:31 PM.
Now hopefully everybody will understand why Ricky "The Hitman" Hatton never gave him a chance. He's an arrogant, self-hyped fighter... Khan would've knocked him out anyway...
By A. Baazab. Posted August 2 2009 at 1:12 PM.