LONDON — After nearly a year of global travel and dozens of matches, most of Novak Djokovic's rivals are complaining about their mental and physical fatigue.
Not the Serb. He thrives in the rigors of a tough autumn finish.
Unbeaten since losing the U.S. Open final to Rafael Nadal, Djokovic joined Nadal in the ATP World Tour Finals semifinals, overcoming Juan Martin del Potro on Thursday.
Djokovic extended his winning streak to 19 matches with a 6-3, 3-6, 6-3 victory over the hard-hitting Argentine.
Djokovic, undefeated in last year's Finals, improved to 2-0 in Group B after beating Roger Federer in the opener.
Federer made light work of Richard Gasquet for his first round-robin win, beating the Frenchman 6-4, 6-3 to boost his chances of making the semifinals for an 11th time.
Nadal qualified from Group A on Wednesday when he beat Stanislas Wawrinka to clinch the season-ending No. 1 ranking for the third time in his career.