HOUSTON, TX, USA -?As temperatures plummeted to an icy 36°F at the Oracle Challenger Series Houston, qualifier Catherine 'CiCi' Bellis's comeback was heating up with a 6-4, 4-6, 7-6(5) barnburner defeat of No.16 seed Varvara Lepchenko over two hours and 25 minutes to move into the third round.

The unranked 20-year-old ended an 18-month hiatus from the game due to wrist and arm injuries this week, and encountered her sternest test thus far in the form of veteran former World No.19 Lepchenko, whose valiant resistance kept pegging Bellis back to the very end of the all-American derby.

Initially, Bellis was sharper both off the ground and on big points. Taking advantage of a series of Lepchenko forehand errors to break serve in the opening game, the former World No.35 was smart in her point construction and efficient at finishing rallies, particularly with her forehand, as she maintained the lead to take the opening set with a drive volley. Lepchenko, meanwhile, was unable to take her opportunities to get back into the match: the 33-year-old failed to convert her first 12 break point chances, squandering four in the first set and eight more in a titanic second game of the second set before falling behind a set and 2-4 as Bellis nailed a backhand winner.

Lepchenko's fruitlessness could not last, though. With her back to the wall, the World No.167 ground out an extended rally to capture the Bellis serve at last as the younger player's backhand went wide - and suddenly, it was Lepchenko in control as the left-hander roared back to reel off four straight games, levelling the match as Bellis double faulted down set point.

The 2016 Honolulu 125K champion appeared to have righted the ship in the deciding set, finding her first-set form to move up a double break for 4-1 - but as the finishing line neared, Bellis backed off and the valiant Lepchenko took full advantage. Safer rallies to the centre of the court allowed Lepchenko to unleash the weight of her powerful forehand, and the 2014 Seoul runner-up slowly began to chip away at Bellis's lead, saving two match points with brilliant winners as the latter served for the win at 5-4.

With Bellis unable to close the deal a second time at 6-5, the match would be decided by a tiebreak - and this time, it was Bellis's turn to mount the comeback, overturning a 1-3 deficit to capture her fourth match point as a Lepchenko backhand went long.

Up next for Bellis will be No.3 seed Kirsten Flipkens, who also came through an epic: a fabulous contest of finesse against 18-year-old Hailey Baptiste in which both players were unafraid to serve-and-volley, carve up every angle of the court with biting slices or flick sudden and perfectly executed dropshots. The American teenager, who announced herself by stunning Madison Keys in her WTA main draw debut in Washington this July, would hold a 5-2 third-set lead today - but Flipkens' experience won out in the end as the 33-year-old Belgian came back to win 3-6, 6-3, 7-6(4) in two hours and five minutes.

Elsewhere, held-over first-round matches saw two more American comebacks gain some momentum: wildcard Bethanie Mattek-Sands posted just her fourth singles victory of the year 6-3, 6-4 over Danielle Lao, while former World No.64 Irina Falconi, who resumed her career in September after a 12-month hiatus, upset Pan-American Games gold medallist Nadia Podoroska 6-4, 6-1. Serbian wildcard Katarina Jokic, the NCAA Championships runner-up and University of Georgia student, also posted her first win at this level, beating lucky loser Kayla Day 6-4, 6-2.

Meanwhile, second-round action saw No.13 seed Ann Li continue her steady progress, with the 2017 Wimbledon junior finalist taking an all-teenage tilt over Serbia's Olga Danilovic 6-1, 7-5. The 19-year-old American, who has reached five ITF World Tennis Tour finals and raised her ranking from World No.310 to World No.147 this season, displayed more court awareness and consistency than her opponent, repeatedly out-manoeuvring the 18-year-old Danilovic as she sped into a 6-1, 5-2 lead - but the 2018 Moscow River Cup champion made a late surge, belatedly hitting her spots with her power and forcing a nail-biting dénouement which Li took on her fourth match point.

Wildcard CoCo Vandeweghe's comeback from a foot injury that kept the former World No.9 out of action for nine months until returning in July, also scored an upset, defeating No.15 seed Sachia Vickery 7-5, 6-3, while former World No.5 Eugenie Bouchard posted consecutive match wins for the first time since her January run to the Newport Beach 125K quarterfinals after No.8 seed Francesca Di Lorenzo retired trailing 6-2, 3-1.