David Goffin banked a shock victory over Rafael Nadal to begin his quest at the 2017 ATP World Tour Finals in surprising fashion, winning 7-6(5), 6-7(4), 6-4 in the Pete Sampras Group.

Nadal fought back from a set down while still on the comeback from injury to see out three sets, but his stamina eventually wilted as Goffin ran out a deserved victor in his debut appearance at this tournament.

Grigor Dimitrov also won his first match of the competition and edged fourth seed Dominic Thiem 6-3, 5-7, 7-5 in a tight encounter to give his campaign at London's O2 Arena an early boost.

Roger Federer will be back in action on Tuesday in the hopes he can best Boris Becker Group rival Alexander Zverev, while Jack Sock—who lost to the Swiss on Sunday—will look to gain some momentum against Marin Cilic.

Read on for a roundup of Monday's ATP World Tour Finals action, complete with a look toward Tuesday's schedule in the English capital.

Monday's Results

Grigor Dimitrov bt. Dominic Thiem: 6-3, 5-7, 7-5

David Goffin bt. Rafael Nadal: 7-6(5), 6-7(4), 6-4

Tuesday Schedule

Marin Cilic vs. Jack Sock, 2 p.m. GMT/9 a.m. ET

Roger Federer vs. Alexander Zverev, 8 p.m. GMT/3 p.m. ET

Seventh seed Goffin may have been looked upon as one of the outsiders at the ATP World Tour Finals prior to Monday's win, but a courageous display against Nadal will have quickly improved his odds at the indoor tournament.

The first Belgian ever to feature at the ATP World Tour Finals was unnerved in his task and was first to break Nadal in the first set, where the Spaniard was guilty of too many unforced errors—albeit his 16 was still fewer than Goffin's 19, per FlashScore.com.

Nadal entered this year's tournament amid concerns over the health of his knee, per BBC Sport, and despite staying alive by breaking back immediately at the end of the first set, Goffin took the opener via tiebreaker, via Tennis TV:

Tennis TV @TennisTV

🚨 Upset alert 🚨 Goffin wins the first set 7-6(5) against Nadal. #NittoATPFinals

Tennis writer Jose Morgado marked this as a career first for Goffin:

José Morgado @josemorgado

That was the first set Goffin ever won against Nadal. Rafa not at 100% as expected, but competing (as always)

If Nadal's confidence looked to be dented in the first set, it certainly was in the second, where he seemed to struggle for motivation at times and his opponent conceded just one point across his first four serving sets.

Again, the pair traded breaks as Nadal stumbled to trail 5-3 before pulling back level at five games apiece in the second set. Goffin restored a one-game lead as signs of stress flashed across Nadal's face, but it wasn't enough to prevent the world No. 1 forcing a second tiebreak.

Nadal responded to falling 2-0 behind in the tiebreak by winning six points in succession, eventually tying on one set apiece after a 7-4 triumph, and tennis writer David Law summed up his second-set heroics:

David Law @DavidLawTennis

Nadal fighting spirit in a sentence: Guaranteed year-end No.1, dodgy knee, coach doesn’t think he should play, saves 4 match points.

That was where the rebellion ended, however, as he served to 1-1 in the second before back-to-back breaks against him saw Goffin blaze ahead 4-1, a lead even Nadal couldn't wrest back. After the match, Nadal announced he was withdrawing from the tournament, saying he wasn't "ready to play."

ATP Media Info detailed just how much Goffin was made to work for his result, as well as touching upon some of those stars with whom he now sits level:

ATP Media Info @ATPMediaInfo

[7] @David__Goffin earns biggest win of career in #NittoATPFinals opener, defeating World No. 1 @RafaelNadal 7-6(5), 6-7(4), 6-4 in 2 hours, 38 minutes. Goffin, @GrigorDimitrov, @RogerFederer and Alexander Zverev all have 1 win.

Dimitrov's first appearance at the ATP World Tour Finals gave him the desired result, albeit perhaps not with the kind of performance the debutant might have sought in what was an at-times scrappy affair.

Thiem gifted his opponent a crucial point after double-faulting at love-30 down and five games apiece in their third and final set, and Tennis TV provided footage as Dimitrov showed stronger nerve when it mattered:

Tennis TV @TennisTV

Dimitrov delivers… @GrigorDimitrov is victorious in his first #NittoATPFinals appearance, beating Dominic Thiem 6-3 5-7 7-5.

He was ultimately the more deserving victor, and Thiem—appearing at the ATP World Tour Finals for a second successive year—finally lost out by small margins, statistically almost identical to his foe, per the event's official website.

Dimitrov put Thiem under the pressure of eight break points—five of which the Austrian saved—compared to just two of his own, and Morgado nodded to the more difficult tests still to come:

José Morgado @josemorgado

Big win for Dimitrov, despite playing far from his best. Thiem now has to play Nadal & Goffin, both terrible matches for him.

The Bulgarian smiled positively on the occasion and said after his win, per NittoATPFinals.com: "It, for me, was just one of those new experiences that I think I'm going to remember for the rest of my life, I think appreciate for the rest of my life. This is where I always wanted to be at, among the top players, also being able to come through in my first match, it's only positive."

Dimitrov will now look to extend his dream start to his debut performance at the O2.