NEWPORT BEACH, CA, USA -- No.7 seed Ajla Tomljanovic of Croatia became the only seed to make it to the semifinals of the Oracle Challenger Series event in Newport Beach on Friday, but it almost didn't turn out that way.

Tomljanovic had to mount a stirring comeback before advancing to the final four of the WTA 125K Series event over American Nicole Gibbs, 4-6, 7-5, 6-2. Gibbs served for the match at 6-4, 5-3, before Tomljanovic was able to rebound and take home a resurgent victory in exactly two hours.

Gibbs was hitting strongly and confidently in the first set, breaking Tomljanovic both times she had chances, while fending off three of the four opportunities the Croatian had on the American's serve. Gibbs broke to claim the opener after moving forward to hit a strong forehand, which Tomljanovic sent long on the reply.

A forehand winner by Gibbs on break point gave her the early break in the second set, to lead 2-0. Both players held serve routinely from there until Gibbs had the opportunity to serve for the match at 5-3. There, the American was broken at love, hitting a backhand miscue into the net down break point to let Tomljanovic get back on serve.

Tomljanovic grabbed the momentum, holding easily for 5-5, then breaking Gibbs for a second consecutive time behind good all-court play and some luck from the netcord, pulling Gibbs out of position for her shots. In the next game, a solid serve on Tomljanovic's second set point was returned into the net, completing a four-game run by the seventh seed to tie the match at one set apiece.

The pair exchanged breaks to start the decider, but once Tomljanovic claimed the break advantage again at 2-2 with a backhand passing winner, it appeared inevitable that the steely Croatian would move past the frustrated American.

Serving for the match at 5-2, Tomljanovic had to outlast stern resistance from Gibbs, as the American continued to deploy strong defense and force Tomljanovic into errors, garnering six break points in a marathon 21-point game. But Tomljanovic finally prevailed on her third match point with a winner off her forehand, her most effective shot.

Tomljanovic will take on another American in the semifinals, as wild card Danielle Collins outlasted South Korea's Jang Su Jeong, 6-0, 3-6, 6-4, in two hours and 12 minutes, to take her place in the last four. Collins won 65 percent of points on her first serve, while Jang could only claim 48 percent of her first serve points, and will also rue six double faults in the match.

Collins never faced a break point in the first set en route to the bagel, but Jang picked up her game in the second set, breaking Collins four times to knot the match at one set all.

Collins recovered to serve for the match at 5-2 in the third set, but Jang broke with a backhand winner to push the set to 5-3, and then saved two match points on her serve in the next game to edge to 5-4. But Collins had no problems in her second chance to close out the match, winning on her third match point when a Jang backhand sailed long.

It will be an all-qualifier affair in the other semifinal, as Russia's Sofya Zhuk will go up against Mayo Hibi of Japan. 18-year-old Zhuk, the 2015 Wimbledon juniors singles champion, took out Jil Teichmann of Switzerland, 6-1, 7-5, in 74 minutes.

The wily all-court play of Hibi led her to another upset of a seeded player, as she got past the highest-seeded player left in the draw, No.6 seed Sachia Vickery of the United States, 6-4, 6-2. Hibi ousted No.2 seed Alison Riske on the first day of main draw play.