SAN DIEGO – With mounting bills, a struggling chiropractic business, a third child on the way, and his UFC career in jeopardy, Dr. Jacob Volkmann picked up a victory when it mattered the most.

Facing well-rounded British welterweight Paul Kelly, Volkmann dominated on the ground and ultimately scored a unanimous-decision victory.

The fight capped off the untelevised preliminary-card action of UFC on Versus 2, which took place at the San Diego Sports Arena in California.

It was a pivotal win for Volkmann, who lost his first two UFC fights but surprisingly got a third booking. He then narrowly edged Ronny Torres for a split-decision win at UFC Fight Night 20 and entered tonight's bout in a must-win situation.

Volkmann's ground skills – sharpened at the University of Missouri, where he was a three-time Division 1 All-American wrestler – proved the deciding factor against Kelly, who couldn't overcome his opponent's smothering top game. And even when he tried to escape, Kelly either gave up dominant positions or was forced off to fend off a variety of submission attempts.

Volkmann never got the stoppage, and he nearly took the full brunt of a flying knee in the final minute of the fight. But the dominant performance earned him all three rounds and a unanimous-decision win via scores of 30-27 from the three judges.

After skating on thin ice, Volkmann (11-2 MMA, 2-2 UFC) now finds himself with back-to-back victories and some much-desired job security. Kelly, a veteran of the UFC's overseas shows who was making his American debut, falls to 10-3 overall and 4-3 in the UFC.

After opponent DaMarques Johnson missed weight for their welterweight bout, Matt Riddle made him pay in more than way.

After surrendering 20 percent of his purse to Riddle as punishment for weighing 172 pounds, Johnson then found himself repeatedly on his back. Riddle, a former state high-school wrestling champion, proved apt on his feet but primarily relied on the takedowns for dominant positions. Once there, his ground-and-pound efforts began to pay dividends.

In the second round, Johnson scored with a knee to the face that briefly stunned his opponent. But Riddle got the fight to the mat, again took top position, and then briefly considered a rear-naked choke as he flattened out Johnson. Instead, Riddle postured up and rained down a quick succession of unanswered blows. Referee Jason Herzog halted the action at the 4:29 mark and awarded him the TKO victory.

Riddle (5-1 MMA, 5-1 UFC), a cast member on "The Ultimate Fighter 7," now has won back-to-back fights since the lone defeat (a TKO loss to Nick Osipczak) of his career. Johnson (14-8 MMA, 2-2 UFC), a "TUF 9" runner-up, now has lost three of his past five bouts.

The seemingly perpetually cursed James Irvin found no luck in his return to the light-heavyweight division.

After a failed experiment at middleweight that left the fighter emaciated and sluggish in a UFC on Versus 1 loss to Alessio Sakara, Irvin returned to 205 pounds. But he ran into a buzzsaw in Croatian fighter Igor Pokrajac, who avoided a third consecutive UFC loss with a quick first-round knockout victory.

After a brief ground skirmish, Pokrajac unloaded more than a dozen straight punches that left his opponent dazed and wobbly. Irvin tried to return fire, but Pokrajac dragged him to the mat, took his back, flattened him out and secured a fight-ending rear-naked choke at the 2:29 mark of the opening round.

The victory moves Pokrajac to 22-7 (1-2 UFC) and undoubtedly saves his career in the UFC. Irvin (14-7 MMA, 4-6 UFC), who's lost three straight fights, now faces an uncertain future in the promotion.

In a bout that marked referee "Big" John McCarthy's return to the UFC octagon, former WEC light-heavyweight champion Brian Stann's middleweight debut proved a successful one as persistence paid off and rewarded him with a third-round submission victory over Mike Massenzio.

Stann, who was fighting for the first time since a February decision loss to Phil Davis at UFC 109, found an evenly matched opponent and split the first two rounds with Massenzio, who frequently scored takedowns throughout the fight. However, an action-packed third round benefited Stann, who fired off multiple triangle-choke attempts before finally securing a successful one on his third try. Massenzio, who was wobbled by a punch at the beginning of the round, tapped out at the 3:10 mark.

The victory comes at a vital time for Stann (9-3 MMA, 3-2 UFC), who releases his autobiography, "Heart for the Fight," next month. Massenzio, fighting for the first time in nearly two years due to injuries, drops to 11-4 (1-2 UFC) and could be released from the UFC following back-to-back losses.

In a lightweight bout that featured one of the most impressive promotional debuts of late, undefeated Brazilian and UFC newcomer Charles Oliveira made quick work of fellow lightweight Darren Elkins and posted a swift submission victory.

Elkins struck first after he picked up and slammed his opponent to the mat. But Oliveira quickly slapped on a triangle choke, maneuvered to halt an escape attempt, alertly readjusted to secure an armbar, and ultimately forced a tap-out from the submission a mere 41 seconds into the fight.

The lightning-quick ground attack earned the UFC rookie a rousing ovation from the San Diego crowd and moves his overall record to 13-0 (1-0 UFC). Elkins, who earned a TKO victory at the first UFC on Versus event when opponent Duane "Bang" Ludwig severely broke his ankle while bracing for a takedown, falls to 12-2 (1-1 UFC).

In the night's opening bout, veteran middleweight Rob Kimmons used multiple takedowns, submission attempts, and sporadic but effective clinch work to top former WEC fighter Steve Steinbeiss via a close but unanimous decision.

Steinbeiss fended off many of the takedowns or quickly returned to his feet early in the early rounds, but the continued assault slowly wore him down as the fight continued. In the third and deciding round, Kimmons scored a late takedown, followed it with ground and pound, and then popped Steinbeiss with a knee to the head in the final seconds. The effort earned him the round and the decision victory via 29-28 scores on all three judges' cards.

The decision was met with a mixed reaction from fans, many of whom felt Steinbeiss won the first two rounds of the fight. (MMAjunkie.com scored it 29-28 in Steinbeiss' favor.)

Regardless, Kimmons (23-5 MMA, 3-2 UFC), who suffered a TKO loss to Jorge Rivera at UFC 104, picks up his sixth win in eight fights and avoids the first back-to-back losses of his career. Steinbeiss (4-4 MMA, 0-2 UFC), meanwhile, drops his third straight fight – all under the Zuffa LLC banner – and faces a potential UFC release.

SEE ALSO: UFC on Versus 2 main-card recap: Jones TKOs Matyushenko, Okami edges Munoz

PRELIMINARY-CARD RESULTS

•Jacob Volkmann def. Paul Kelly via unanimous decision (30-27, 30-27, 30-27)
•Matt Riddle def. DaMarques Johnson via TKO - Round 2, 4:29
•Igor Pokrajac def. James Irvin via submission (rear-naked choke) - Round 1, 2:29
•Brian Stann def. Mike Massenzio via submission (triangle choke) - Round 3, 3:10
•Charles Oliveira def. Darren Elkins via submission (armbar) - Round 1, 0:41
•Rob Kimmons def. Steve Steinbeiss via unanimous decision (29-28, 29-28, 29-28)