The Gelsenkirchen-based club had a five-point lead over Dortmund after winning their Ruhr derby on Oct. 20 but finds itself one behind the defending champions after Saturday's 1-1 draw with Eintracht Frankfurt.
For some fans, that's even worse than falling 10 points behind league leader Bayern Munich.
“It's always annoying to be overtaken,” Schalke midfielder Julian Draxler said.
Dortmund hosts Fortuna Duesseldorf, also Tuesday, with Sebastian Kehl, Mario Goetze and Ilkay Guendogan all in doubt.
Dortmund coach Juergen Klopp said Monday he could only decide on their participation before the game.
“Duesseldorf have developed into a really good counterattacking side,” Klopp said. “We'll have to patient but determined.”
Bayern visits Freiburg on Wednesday, before the top two meet in Munich on Saturday.
Also Wednesday, Borussia Moenchengladbach hosts Wolfsburg, Augsburg visits Stuttgart, Werder Bremen plays Bayer Leverkusen and struggling Nuremberg hosts third-from-bottom Hoffenheim.
Hoffenheim coach Markus Babbel will find his position under further scrutiny after the club's worst start to a Bundesliga campaign since securing promotion in 2008.
“Right now I'm a bad coach because the results aren't going our way,” Babbel said after Sunday's 2-1 loss to Leverkusen.
Hannover hosts Greuther Fuerth on Tuesday, as Mainz visits Frankfurt.
Schalke's inconsistency has thwarted attempts to keep up with Bayern, which has claimed more points (34) than any other side after 13 games in a Bundesliga season.
Bayern has a squad that is the envy of the Bundesliga, with record-signing Javi Martinez, Switzerland winger Xherdan Shaqiri and established stars Franck Ribery and Arjen Robben all competing with Germany internationals Toni Kroos, Bastian Schweinsteiger and Thomas Mueller for places in midfield.
Bayern coach Jupp Heynckes can rotate his players to handle the challenges of three competitions, but Huub Stevens has no such luxury at Schalke and has fielded largely the same lineup from game to game.
Schalke's recent dip in form in the Bundesliga - four points from the last four games - came as it secured qualification from its Champions League group with a game to spare, signaling a lack of strength in depth needed to maintain a serious challenge across multiple competitions over a season.
Schalke plays Mainz in the third round of the German Cup on Dec. 18.
Against Frankfurt, Stevens made only one change from the side that beat Olympiakos 1-0 three days before, with Japan defender Atsuto Uchida coming in for the suspended Kyriakos Papadopoulos.
He paid tribute to his players for their performance against Frankfurt, coming after the game against Olympiakos which “took a lot out of them.”
Despite acknowledging their tiredness, Stevens' only substitution against Frankfurt was Finland striker Teemu Pukki for Lewis Holtby in the second half, betraying a lack of faith in a bench that includes attacking players like Tranquillo Barnetta, Ciprian Marica and Chinedu Obasi.
Heynckes was able to bring on Germany striker Mario Gomez, who scored seconds later to round off a 5-0 win for Bayern over Hannover on Saturday. Gomez's absence following ankle surgery in August was barely noticed with Mario Mandzukic and Claudio Pizarro chipping in with 18 goals across all competitions, including the German Supercup.
Bayern has scored 38 goals in all in the Bundesliga and conceded only five, also a record for this stage in the season.
Hamburg has been depleted by injuries ahead of Schalke's visit. The side will be hard hit by Rafael van der Vaart's absence after he tore a muscle in his right thigh in Friday's 2-0 loss at Duesseldorf.
Hamburg has failed to score in the Bundesliga without Van der Vaart on the field this season. The Dutchman, who has contributed one goal and five assists, has been ruled out for at least three weeks.
Son Heung-min is also out with a muscle strain in his right thigh, while Tomas Rincon will play with a splint after tearing a tendon in his right ring finger. The Venezuela midfielder will have an operation after the game.
“We'll try to hurt Schalke with the team we have, and to win the game,” Hamburg coach Thorsten Fink said.
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