Austrian Dominic Thiem defeated Spain's Rafael Nadal in straight sets on Tuesday to remain undefeated at the ongoing Nitto ATP Finals tournament in London.

Thiem beat the 20-time Grand Slam champion 6-7, 6-7 in a close encounter which edged him closer to the semi-finals of the event.

The first set went with serve forcing it into a tie-breaker. After a topsy-turvy 10 games in the tie-breaker, Nadal took a 6-5 lead after Thiem lost serve but he then failed to convert his first set point, and even the one that followed while leading 7-6. Thiem then fought back and capitalized on Nadal's failure to take a 7-8 lead after which he converted his first set point to win the tie-break 7-9.

The second set went with serve until Thiem dropped serve in the seventh game allowing Nadal to lead 4-3. The 2020 U.S. Open champion then broke Nadal immediately to level the game at 4-4. However, at 5-5, Thiem missed three match points as Nadal eventually forced the set into another tie-breaker.

This time round, Thiem held his nerve only dropping serve once as he converted his second match point during the tie-break to win it 4-7 and claim the second set.

It was the first time that Thiem had recorded back-to-back wins over Nadal for the first time in his career.

Thiem could progress to the semi-finals if Greece's Stefanos Tsitsipas defeats Russia's Andrey Rublev later on Tuesday. Thiem is scheduled to play Rublev, who is making his debut at the event, on Thursday.

(With input from agencies)