Watch the WTA finals LIVE on beIN SPORTS 

Halep was forced to hang in the contest as Andreescu breezed her way to the opener and was afforded match point before the second set went to a tiebreak.

But 28-year-old's trademark tenacity gradually pulled the argument in her favour as her ebullient foe was compromised by back problems down the stretch.

Andreescu had spoken of her excitement at facing Halep for the first time in her career but this did not extend to reverence of any sort during the contest.

The 19-year-old's blistering ground strokes were to the fore, but she also played with a variety that never allowed Halep to settle during an opening set she won in 39 minutes.

Halep secured an early break in the second but ceded it when Andreescu pulled level at 4-4 – the game after the Wimbledon champion was seemingly irritated by her opponent pulling off a through-the-legs tweener shot.

Andreescu was unable to take a match point at advantage on the Halep serve and the Romanian raced into a 3-0 lead in the tie-break.

Halep then converted her third set point, after which Andreescu received treatment for the first time.

When the trainer came on again immediately before a break for Halep, Andreescu's race appeared to be run – only for her to stunningly get back on terms to 15.

A third straight break followed, however, and Halep would not be denied, clattering home the first of three match points to conclude a captivating two-and-a-half hours.

Up next for Halep in the Purple Group is Elina Svitolina, who began the defence of her title with a straight-sets win over Karolina Pliskova, a titanic first-set tie-break proving pivotal to a 7-6 (14-12) 6-4 win.

Pliskova broke in the opening game but too often produced lackadaisical tennis as she failed to build upon the advantage, not least when Svitolina took the second of two break points for 2-2.

A breaker of epic proportions was eventually taken by Svitolina, the frontrunner throughout converting a seventh set point having only had to save one conceded to Pliskova.

The Czech won three straight games from 2-0 down in the second but Svitolina broke back for 3-3 and came through some stern examinations of her serve before sealing victory when Pliskova sent a return into the net.