Rafael Nadal kept alive his hopes of a first Men's Professional Tennis (ATP) Finals title on Thursday, beating defending champion Stefanos Tsitsipas of Greece 6-4, 4-6, 6-2 in London to qualify for the last four.

The Spaniard, whose haul of 86 singles titles includes just one on indoor hard courts, is chasing the biggest prize missing from his resume at the eight-man elite event.

The 34-year-old has qualified for the ATP Finals for a record 16 consecutive years but has been forced to withdraw on six occasions.

The second seed won his opening round-robin match against Andrey Rublev of Russia but defeat against Dominic Thiem of Austria meant he faced a shootout against 2019 champion Tsitsipas to reach the semi-finals.

Both players were rock solid on serve until Nadal earned the first two break points of the set in the seventh game, which he failed to convert.

"In general this was a very positive match for me," said Nadal, who despite winning 20 Grand Slam titles has only two runners-up places to show for his previous nine appearances at the ATP Finals. "Excited to be in the semi-finals."

Earlier on Thursday, Thiem, who had already qualified for the semi-finals, slumped to a 6-2, 7-5 defeat against debutant Rublev in a dead rubber.

The Russian seventh seed raced into a 4-0 lead in the first set and defied a fightback from the US Open champion in the second set to seal the match, breaking the Austrian four times in total.

"With the fact that I was already qualified in the back of my mind, it was maybe difficult today to keep that intensity alive," said Thiem, who had already clinched top spot in Group London 2020.