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It was the longest WTA Finals showpiece on record, and ended with a dominant tiebreak for Gauff after she saw off a third-set surge by the Chinese star, who scored two breaks against the American and served for the match at 5-4 before succumbing to the pressure.

Gauff win $US4,805,000 for her week in Riyadh and also claims a season-ending world No.3 ranking for the second straight year.

Zheng looked a dominant force in the opening set, and broke Gauff to love for a 5-3 lead.

The Olympic champion survived a break point against her own serve before seeing out the set as Gauff went 0-for-5 on break points in the the first set.

But the American found her aggressive return game in the second set and from 3-1 down, she broke Zheng three times in succession to level matters at a set apiece.

The third set was less predictable as both players pushed themselves to their limits in search of victory.

Gauff again came back from an early break down, but Zheng reclaimed the lead and served for the match at 5-4. 

While Zheng struggled to subdue Gauff, she did save two match points on her own serve at 5-6 down to push things to a tiebreak, but Gauff wasn't giving in.

Gauff surged into a 6-0 lead in the tiebreak, allowing a brief glimmer of hope for Zheng that never took hold before converting her fifth match point to seal the victory.