London - Defending champion Novak Djokovic booked his place in the semi-finals of the ATP World Tour Finals with a 6-3, 3-6, 6-3 victory over Juan Martin del Potro on Thursday.
After Roger Federer defeated Richard Gasquet earlier in the day, Djokovic knew any win against the Argentine fourth seed would guarantee his qualification from Group B with one match still to play.
And the world number two seized his chance, surviving a tough test lasting nearly two hours to extend his current winning run to 19 matches.
Del Potro can still make the last four as well, but he will have to beat Federer in their match on Saturday.
"With his serve it's definitely a big advantage if I can get a break and then close it out," Djokovic said.
"Thankfully I managed to do that. I'm very glad I could win this match and make it to the semi-finals."
Djokovic discovered on Wednesday that his bid to finish the season as world number one had failed after Rafael Nadal wrapped up top spot in the year-ending rankings.
But he showed no signs of letting that disappointment affect him as he continued his bid to retain the title at the prestigious season-ending event at London's O2 Arena.
Four months ago, on a sweltering July afternoon, Djokovic out-lasted del Potro in an epic encounter that took nearly five hours and finished as the longest semi-final ever played at Wimbledon.
This was their first meeting in London since then and Del Potro took the attack to Djokovic in the Serb's opening service game, securing two break points.
But, as was so often the case when they met at Wimbledon, Djokovic was able to get out of trouble with peerless ability to turn defence into attack in one motion.
Djokovic was unable to convert three break points in the fourth game, but simply came back even harder in del Potro's next service game and this time he did break to open up a 4-2 lead.
That was enough to effectively seal the first set and put the two-time Tour Finals champion within one set of a place in the last four.
Del Potro refused to accept his fate however and, hitting a rich vein of form, he pressed Djokovic into conceding two break points in the sixth game of the second set, converting the first thanks to a fortunate net-cord before going on to level the match.
But Djokovic had beaten the former US Open winner in 10 of their previous 13 meetings and it wasn't long before he was back on top.
The Australian Open champion had to stave off two break points early in the final set, but he gradually turned the tide and landed the decisive break for a 4-2 lead before serving out the win.