Victory meant US Open finalist Del Potro has sealed qualification for next month’s season-ending ATP Finals in London, joining Rafa Nadal, Novak Djokovic and Roger Federer. After cruising in the first set, Argentina’s Del Potro faced a stern test in the next as Khachanov began to find his rhythm. The Russian looked set to drag the contest into a decider but world number four Del Potro held his nerve in the tiebreak to close out the victory with an ace on match point. Del Potro next takes on Serbia’s Filip Krajinovic in the last eight after the Serbian overcame Spaniard Feliciano Lopez 7-6(5) 6-3. Del Potro beat Krajinovic in their only previous meeting in Miami earlier this season. Krajinovic’s compatriot Lajovic took advantage of Dimitrov’s inconsistent serving to knock the world number eight out of the tournament in an hour and 50 minutes. “The first set was hard to play because we had a shadow on the court… in the end it was a dog fight,” Lajovic said after his second win over a top-10 player. The 28-year-old Serbian next meets fifth seed Kyle Edmund after the Briton overcame Italy’s Matteo Berrettini 7-5 6-7(2) 7-5 in a tightly contested match lasting nearly three hours.